About if contains comma expression in JavaScript

Sometimes you will see English commas "," in the if statement in JavaScript. This is actually a comma expression. In the if condition, only the last expression plays a role.

Consider the following example:

let a = 1,

	b = 2,

	c = 3;

// if only checks the last expression if (a == 10, b == 20, c == 3) {

	console.log("c == 3");

} else {

	console.log("c != 3")

}



// Console output:

// c == 3

There are 3 expressions in the above if statement, the first two are not true, only the last one is true, and only the last one is tested in if statement, so the result is true .

Although if only checks the last expression, the previous expressions will also be executed.

Please refer to the following code:

let a = 10,

	b = 20,

	c = 30;

// if only checks the last expression, but the previous expressions will also be executed if (a = 1, b = 2, c == 3) {

	console.log("c == 3");

} else {

	console.log("c != 3")

}

// Console output:

// c != 3



console.log(a, b); 

// Console output:

// 1 2

In the if statement, the last condition c == 3 is not true, so the if statement is false .

But the first two expressions are executed, so finally a=1 , b=2 .

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