JavaScript == vs ===: Complete Guide to Equality Operators

Brief Answer

JavaScript’s equality operators differ in how they handle type comparison: loose equality (==) performs type conversion before comparison, while strict equality (===) requires both value and type to be identical. The strict equality operator is generally recommended for its predictability, while loose equality has specific use cases when intentional type conversion is desired. Performance differences between the operators are typically negligible in real-world applications.

Contents

• What is the fundamental difference between == and ===?
• How does type coercion work with the == operator?
• When should you use == versus ===?
• Are there performance differences between == and ===?
• What about JSLint’s recommendation to use ===?
• Practical examples and best practices

What is the fundamental difference between == and ===?

The key difference between JavaScript’s equality operators lies in how they handle type comparison:

• Loose equality (==): This operator performs type coercion before making the comparison. It attempts to convert the operands to the same type before evaluating equality.

• Strict equality (===): This operator does not perform type coercion. It returns true only if both operands have the same value and the same type.

This fundamental distinction leads to different behaviors in various comparison scenarios:

// Example showing the difference
console.log(5 == "5");  // true (type coercion occurs)
console.log(5 === "5"); // false (types are different)

console.log(0 == false);  // true (false is coerced to 0)
console.log(0 === false); // false (types are different)

console.log(null == undefined);  // true (special case)
console.log(null === undefined); // false (types are different)

The strict equality operator (===) is generally considered more predictable because it doesn’t have the hidden behavior of type conversion, which can lead to unexpected results in complex comparisons.

How does type coercion work with the == operator?

Type coercion with the == operator follows a set of complex rules that can sometimes lead to surprising results. Understanding these rules is essential for writing predictable code when using == comparisons.

Here’s how type coercion generally works:

1. If both operands are of the same type, == behaves the same as ===, comparing the values directly.

2. If the operands are of different types, JavaScript attempts to convert them to a common type using these rules:

   • Comparing numbers and strings: The string is converted to a number.
   javascript

   console.log(5 == "5"); // true
   console.log(5 == "05"); // true
   

   • Comparing numbers and booleans: The boolean is converted to a number (true becomes 1, false becomes 0).
   javascript

   console.log(1 == true); // true
   console.log(0 == false); // true
   console.log(2 == true); // false
   

   • Comparing strings and booleans: Both are converted to numbers.
   javascript

   console.log("1" == true); // true
   console.log("0" == false); // true
   

   • Comparing objects with primitives: The object is converted to a primitive value (using its valueOf() or toString() method).
   javascript

   console.log([1] == "1"); // true
   console.log(new Date() == "Wed Jul 21 2021 12:00:00 GMT-0400 (Eastern Daylight Time)"); // true
   

   • Special cases:
     javascript

     console.log(null == undefined); // true
     console.log(null == false); // false
     console.log(undefined == false); // false
     console.log(NaN == anything); // false, including NaN == NaN
     console.log({} == {}); // false (different object references)
     

The type coercion rules can be counterintuitive, which is why many developers prefer === to avoid these hidden conversions. For example:

// These comparisons might seem surprising:
console.log("" == 0); // true
console.log("0" == 0); // true
console.log("0" == false); // true
console.log(null == undefined); // true
console.log([] == ![]); // true (because ![] is false, and [] is coerced to 0)

When should you use == versus ===?

The choice between == and === depends on your specific use case and whether you want type conversion to occur.

Use === (strict equality) when:

1. You want to ensure both value and type are the same: This is the most common case in JavaScript programming. === provides more predictable and reliable comparisons.

2. Comparing primitive values directly: When you’re sure about the types of both operands and want a straightforward comparison.

3. When working with object references: For determining if two variables reference the same object in memory.

4. When comparing with null or undefined: Using === helps distinguish between these values.

// Good use cases for ===
if (userInput === "") {
  // Only true if userInput is an empty string, not falsy values
}

if (value === null) {
  // Explicitly checking for null, not undefined or other falsy values
}

if (obj1 === obj2) {
  // Checking if they refer to the exact same object
}

Use == (loose equality) when:

1. You intentionally want type conversion: In cases where you’re checking for “truthiness” or “falsiness” and want to handle multiple types that evaluate to the same boolean.

2. Comparing with null or undefined: The special case where null == undefined returns true can be useful when you want to treat these values as equivalent.

3. Some specific DOM comparisons: When working with DOM APIs that might return values of different types but represent the same logical state.

4. Legacy code compatibility: When working with older codebases or APIs that rely on type coercion.

// Good use cases for ==
if (value == null) {
  // True for both null and undefined
}

if (input == "") {
  // True for empty string, null, undefined, false, 0, etc.
}

// Checking for "empty" values in a more permissive way
if (response == null) {
  // Handles both null and undefined as "no response"
}

Are there performance differences between == and ===?

Regarding performance, there are a few key points to consider:

1. When types are the same: If both operands already have the same type, === is generally faster because it doesn’t need to perform type checking and conversion. However, the difference is typically negligible in most applications.

2. When types are different: == has to perform type conversion, which involves additional steps compared to ===. In these cases, === will be faster because it can immediately return false when types differ.

3. No performance benefit when no type conversion occurs: If you’re comparing values of the same type (e.g., two numbers or two strings), there’s no meaningful performance difference between == and ===. JavaScript engines are highly optimized, and the performance difference would be minimal and likely not noticeable in most applications.

4. Microbenchmarking considerations: While you might find microbenchmarks showing a slight performance advantage for === in certain scenarios, these differences are rarely significant enough to impact real-world application performance. The primary consideration should be code clarity and correctness rather than micro-optimizations.

// Performance test (not conclusive, just for illustration)
const start = performance.now();
for (let i = 0; i < 1000000; i++) {
  5 === "5"; // Strict comparison
}
const end = performance.now();
console.log(`=== took: ${end - start}ms`);

const start2 = performance.now();
for (let i = 0; i < 1000000; i++) {
  5 == "5"; // Loose comparison
}
const end2 = performance.now();
console.log(`== took: ${end2 - start2}ms`);

In practice, the performance difference between == and === is not a significant factor in choosing which operator to use. The more important considerations are code clarity, predictability, and avoiding bugs related to unexpected type coercion.

What about JSLint’s recommendation to use ===?

JSLint (and its successor JSHint) was created by Douglas Crockford to enforce code quality and consistency in JavaScript. The recommendation to use === instead of == is based on several factors:

1. Avoiding common bugs: Many JavaScript bugs arise from unexpected type coercion with ==. By encouraging ===, JSLint helps developers avoid these pitfalls.

2. Code clarity and predictability: === makes the developer’s intent clearer - you’re checking for both value and type equality.

3. Consistency: Enforcing === usage creates more consistent codebases, making them easier to read and maintain.

4. Explicit type handling: === forces developers to be explicit about types, which can lead to better type handling practices.

The recommendation to replace idSele_UNVEHtype.value.length == 0 with idSele_UNVEHtype.value.length === 0 is a good example of this principle. In this case, the comparison is checking if a length value is zero. Using === ensures that you’re explicitly checking for the numeric zero, rather than allowing any falsy value (like an empty string, null, or undefined) to pass the check.

Modern tools like ESLint continue this tradition with rules like eqeqeq (which enforces the use of === over ==) by default. This has become a best practice in the JavaScript community.

While there might be rare cases where == is more appropriate, the default should be to use === unless you have a specific reason not to. This aligns with the principle of “make it explicit” in code - your intentions should be clear from the code itself.

Practical examples and best practices

Let’s examine some practical examples that illustrate when to use == versus ===:

Example 1: Form validation

// Using === for explicit type checking
function validateEmail(email) {
  if (email === "") {
    return "Email cannot be empty";
  }
  if (typeof email !== "string") {
    return "Email must be a string";
  }
  // Additional validation logic...
}

// Using == for more permissive "empty" check
function isEmpty(value) {
  if (value == null || value == "") {
    return true; // Also returns true for null, undefined, "", 0, false
  }
  return false;
}

Example 2: Conditional rendering

// Using === for precise type checking
function renderButton(status) {
  if (status === "active") {
    return "<button class='active'>Active</button>";
  }
  if (status === "inactive") {
    return "<button class='inactive'>Inactive</button>";
  }
  return "<button>Unknown</button>";
}

// Using == to handle different string representations
function getStatusColor(status) {
  if (status == 0) { // Will match 0, "0", false, etc.
    return "red";
  }
  if (status == 1) { // Will match 1, "1", true, etc.
    return "green";
  }
  return "gray";
}

Example 3: Array operations

// Using === for identity comparison
const array1 = [1, 2, 3];
const array2 = [1, 2, 3];
const array3 = array1;

console.log(array1 == array2); // false (different references)
console.log(array1 === array2); // false (different references)
console.log(array1 == array3); // true (same reference)
console.log(array1 === array3); // true (same reference)

// Using == with indexOf
const items = ["apple", "banana", "orange"];
if (items.indexOf("apple") >= 0) { // Strict comparison is fine here
  console.log("Apple found");
}

Best practices summary:

1. Default to ===: Use strict equality by default unless you have a specific reason to use loose equality.

2. Be explicit about types: If you want to allow type conversion, make it clear in your code comments or variable names.

3. Handle special cases intentionally: When you need to treat null and undefined as equivalent, document this decision.

4. Avoid == with objects and arrays: These rarely behave as expected due to reference comparison and type coercion.

5. Consider your code’s context: In some legacy codebases or when working with certain APIs, == might be more appropriate, but document why.

Conclusion

JavaScript’s equality operators serve different purposes, and understanding when to use each is crucial for writing robust code.

Key takeaways:

• === (strict equality) compares both value and type without conversion, providing more predictable results.
• == (loose equality) performs type coercion before comparison, which can lead to unexpected behavior.
• Performance differences between the operators are generally negligible and shouldn’t be the primary factor in your decision.
• JSLint and other linting tools recommend === to avoid common bugs related to type coercion.
• Best practice is to default to === unless you specifically need type conversion in your comparison.

In your specific case with JSLint suggesting to replace idSele_UNVEHtype.value.length == 0 with === 0, this is good advice. The strict comparison ensures you’re explicitly checking for the numeric zero, avoiding any potential issues with type coercion.

By following the principle of “explicit is better than implicit” in your equality comparisons, you’ll write clearer, more maintainable JavaScript code that’s less prone to subtle bugs.