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Introduction and usage examples of comma operator in JavaScript_javascript skills

WBOY
Release： 2016-05-16 16:09:30
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There is a js interview question, the question is this: What is the execution result of the following code and why?

Copy code The code is as follows:

var i, j, k;
for (i=0, j=0; i<10, j<6; i , j ) {
k = i j;
}
document.write(k);

The answer is 10. This question mainly examines JavaScript’s comma operator.

The following is MDN’s definition of the comma operator:

The comma operator evaluates two operands (from left to right) and returns the value of the second operand.

According to this definition, it can be expanded:

The comma operator evaluates two or more operands from left to right and returns the value of the last operand.

You can feel the code below:

Copy code The code is as follows:

alert((0, 9));
alert((9, 0));

if (0,9) alert("ok");
if (9,0) alert("ok");

What role does the comma operator play in actual code?

1. Exchange variables, no third variable is needed

Copy code The code is as follows:

var a = "a", b = "b";

//Method 1
a = [b][b = a, 0];

//Method 2
a = [b, b = a][0];

2. Simplified code

Copy code The code is as follows:

if(x){
foo();
return bar();
}
else{
return 1;
}

can be abbreviated as:

Copy code The code is as follows:

return x ? (foo(), bar()) : 1;
Related labels：
javascript comma operator
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