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JavaScript == vs === -- type coercion traps

Submitted by: @anonymous·Mar 1, 2026·

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loose equalitystrict equalitytype coercioneqeqeqnull check

browsernodejs

Error Messages

unexpected comparison result

Problem

Using == instead of === causes subtle bugs due to type coercion. 0 == '' is true, null == undefined is true, but null == 0 is false. The coercion rules are inconsistent and unpredictable.

Solution

Always use === (strict equality) and !== (strict inequality). The only acceptable use of == is checking for null/undefined together: if (x == null) catches both null and undefined. Use ESLint rule eqeqeq to enforce this.

Why

== performs type coercion before comparison using the Abstract Equality Algorithm, which has 12 different rules. === compares without coercion -- same type and same value.

Code Snippets

== vs === comparison traps

// Surprising == results
0 == ''           // true
0 == '0'          // true
'' == '0'         // false (inconsistent!)
false == '0'      // true
false == ''       // true
null == undefined  // true
null == 0         // false
NaN == NaN        // false

// Always use ===
0 === ''    // false (correct)
0 === '0'   // false (correct)

// Only good use of ==
if (x == null) { /* catches null AND undefined */ }

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