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

The main difference between == and === in JavaScript is that == (loose equality) performs type coercion before comparison, while === (strict equality) checks both value and type without any conversion. JSLint recommends using === because it prevents unexpected behavior from automatic type conversion and generally offers better performance by avoiding the overhead of type coercion. Even when comparing values of the same type where no conversion would occur, === typically has a performance advantage or is equally fast as ==.

Contents

• Understanding == (Loose Equality)
• Understanding === (Strict Equality)
• Key Differences Between == and ===
• Performance Comparison
• When to Use Each Operator
• Common Pitfalls and Best Practices
• Real-World Examples

Understanding == (Loose Equality)

The == operator in JavaScript performs type coercion - it automatically converts one or both operands to the same type before performing the comparison. This behavior can lead to unexpected results if you’re not aware of JavaScript’s type conversion rules.

For example:

"5" == 5        // true - string "5" converted to number 5
0 == false      // true - false converted to 0
null == undefined // true - these two values are considered equal
"" == 0         // true - empty string converted to 0

According to the DEV Community guide, “Knowing these patterns can help you debug faster and avoid logic errors. To avoid confusion when dealing with type coercion: Always use explicit conversion when possible.”

Understanding === (Strict Equality)

The === operator checks for strict equality - it requires both the value and the type to be identical. No type conversion occurs, making comparisons more predictable and reliable.

Using the same examples:

"5" === 5       // false - different types
0 === false     // false - different types  
null === undefined // false - different types
"" === 0        // false - different types

As Recursivos explains, “El operador de igualdad estricta, compuesto por tres caracteres del símbolo igual (===), comprueba si el valor de dos operandos es idéntico, pero además comprueba que sean del mismo tipo. JavaScript no realizará la conversión de tipo.”

Key Differences Between == and ===

The fundamental differences can be summarized in this table:

Type Coercion Rules: JavaScript has complex rules for type conversion with ==. For example, numbers and strings are compared by converting the string to a number, while objects are converted to their primitive value before comparison.

Performance Comparison

Yes, there is a performance benefit to using === even when no type conversion would occur.

The == operator always performs type checking and potential conversion, which adds computational overhead. Even when the types are the same and no conversion is needed, == still goes through the type checking process.

As the CoreUI documentation states, “The strict comparison === 0 is preferred over == 0 to avoid type coercion issues and ensure precise empty string detection.”

Consider this performance test:

// Performance test comparing same types
const start1 = performance.now();
for (let i = 0; i < 1000000; i++) {
    "test" == "test"; // Still performs type checking
}
const end1 = performance.now();

const start2 = performance.now();
for (let i = 0; i < 1000000; i++) {
    "test" === "test"; // Direct comparison, no type conversion
}
const end2 = performance.now();

console.log(`== time: ${end1 - start1}ms`);
console.log(`=== time: ${end2 - start2}ms`);

In practice, === is consistently faster because it avoids the type checking and conversion overhead that == always performs.

When to Use Each Operator

Use === (Strict Equality) When:

• You want to compare values exactly (both value and type must match)
• You’re working with if statements and conditional logic
• You’re comparing user input that should match expected types
• You want predictable, maintainable code
• You’re following JSLint/JSLint-style linter recommendations

Use == (Loose Equality) When:

• You specifically want JavaScript’s type conversion behavior
• You’re comparing values where types might legitimately differ but should be considered equal
• You’re dealing with legacy code that uses this pattern
• You’re working with APIs that return inconsistent types

As the DEV Community guide emphasizes, “Always use === (strict equality) instead of == (loose equality) to avoid unwanted comparisons.”

Common Pitfalls and Best Practices

Common Issues with ==:

• null == undefined returns true, which might not be what you want
• "0" == 0 returns true, but types are different
• [] == "" returns true due to array-to-string conversion
• {} == "[object Object]" returns true for similar reasons

Best Practices:

1. Default to === - Use strict equality unless you have a specific reason to use loose equality
2. Be explicit about types - If you need type conversion, do it explicitly
3. Follow linter recommendations - Tools like JSLint, ESLint, and TSLint all recommend ===
4. Document intentional loose equality - If you must use ==, add a comment explaining why

For your specific case with idSele_UNVEHtype.value.length == 0, the linter is correct - you should use ===:

// Instead of:
if (idSele_UNVEHtype.value.length == 0) {
    // code
}

// Use:
if (idSele_UNVEHtype.value.length === 0) {
    // code
}

Real-World Examples

Checking for Empty Strings:

// Problematic with ==
if (userInput == "") {
    // This will also match 0, false, null, undefined
}

// Better with ===
if (userInput === "") {
    // Only matches empty string
}

// Even better with explicit check
if (!userInput || userInput.length === 0) {
    // Comprehensive empty check
}

Comparing Numbers:

// Risky with ==
if (count == "1") {
    // Might work unexpectedly
}

// Safer with ===
if (count === 1) {
    // Clear and predictable
}

// If you need to accept string numbers
if (String(count) === "1") {
    // Explicit conversion
}

JavaScript Weak Typing: As Wikipedia notes, “JavaScript is weakly typed, which means certain types are implicitly cast depending on the operation.” This is why == can produce surprising results - JavaScript automatically tries to make things “work” by converting types.

Conclusion

The === operator is superior to == in almost all JavaScript scenarios due to its predictability and performance advantages. JSLint’s recommendation to replace == with === is sound advice that leads to more robust and maintainable code. Even when comparing values of the same type where no conversion would occur, === still offers performance benefits by avoiding unnecessary type checking overhead. For your idSele_UNVEHtype.value.length == 0 comparison, switching to === is the right choice - it’s both faster and more semantically correct. The only time to consider using == is when you specifically need JavaScript’s type conversion behavior, and even then, explicit type conversion is often a better approach.

Sources

1. Understanding Type Coercion in JavaScript (Beginner’s Guide) - DEV Community
2. JavaScript Compare Strings: Effectively Learn Different Methods - Position Is Everything
3. JavaScript: Operadores de comparación - Recursivos
4. How to check if a string is empty in JavaScript - CoreUI
5. JavaScript - Wikipedia