Understanding Double Structural Equality Operators in JavaScript: What Happens with (a == b == c)?

What does the expression (a == b == c) evaluate to in JavaScript, and why might it lead to confusion?

let a = 5;
let b = 5;
let c = 5;
console.log(a == b == c); // Output: false

In JavaScript, the expression (a == b == c) can lead to confusion due to how equality operators work, especially the chaining of comparisons. The structure evaluates left to right, which can yield unexpected results.

let a = 5;
let b = 5;
let c = 5;
console.log((a == b) && (b == c)); // Output: true

Causes

• The `==` operator checks for equality but performs type coercion when necessary.
• When evaluating (a == b), it produces a boolean value (true or false).
• Then, the comparison of that boolean result with c may yield a non-intuitive result, especially if c is not strictly true or false.

Solutions

• To compare three values for equality, use parentheses to explicitly define the order of operations: (a == b) && (b == c).
• Utilize strict equality (===) instead of loose equality (==) to avoid type coercion issues. It checks both value and type, reducing ambiguity.

Mistake: Assuming (a == b == c) compares all three variables directly.

Solution: Remember it evaluates left to right, so first (a == b) is calculated.

Mistake: Using (==) when you meant to use (===) for strict equality.

Solution: Always consider the type of variables being compared and use strict equality when possible.