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javascripttypesequality

Multiple strict equality comparisons in a single line

I have the following JavaScript code:

var a = 1;
var b = 1;
a === 1        // evaluates to true
b === 1        // evaluates to true
a == b == 1    // evaluates to true
a === b === 1  // evaluates to false

Why does a === b === 1 evaluate to false?

Solution 
  • a == b == 1

    is evaluated as

    ((a == b) == 1)

    Since a == b is true, the expression becomes 

    true == 1

    Since == does type coercing, it converts true to a number, which becomes 1. So the expression becomes

    1 == 1

    That is why this expression is true. You can confirm the boolean to number conversion like this

    console.log(Number(true));
// 1
console.log(Number(false));
// 0

    Similarly,

    a === b === 1

    is evaluated as

    ((a === b) === 1)

    so

    true === 1

    Since === doesn't do type coercion (as it is the strict equality operator), this expression is false.