JavaScript运算符优先级

Question

According to Mozilla , the === operator has higher precedence than the || operator, which is what I would expect.

However this statement evaluates to the number 1, rather than false.

let x = 1 || 0 === 0; // x === 1;

You have to wrap in parens to get a boolean:

let x = (1 || 0) === 0; // x === false;

What gives?

NOTE this is not a dup of this question, which does not have anything about equality operators - JavaScript OR (||) variable assignment explanation

Answer 1

Higher operator precedence is like a parenthesis around the operands.

let x = 1 || (0 === 0);

The second part gets never evaluated, because of the truthy value of 1 .

Answer 2

|| is a short circuit operator and conditions are evaluated from left to right.
So here : left || right left || right , if the left condition is true , the whole condition is evaluated to true and the right one is never evaluated.

Here :

let x = 1 || 0 === 0; // x === 1;

x = 1 assigns 1 to x and the second condition after || is never evaluated as if (1) is evaluated to true .

And here :

let x = (1 || 0) === 0; // x === false;

(1 || 0) is evaluated to true as if (1) is still evaluated to true .
And then true === 0 is evaluated to false .
So x is valued to false .