JavaScript 中的海象运算符等效项

Question

Is there an equivalent of Python's walrus operator ':=' in JavaScript? I know it's possible to do:

for (let i = 0; (result = foo(i)) < N; i++) {
    // Do stuff.
}

but I want result to be constrained to the scope of the for-loop.

Answer 1

You could declare result inside of the for 's scope.

for (let i = 0, result; (result = foo(i)) < N; i++) {
    // Do stuff.
}

Answer 2

Just as an alternative to Nina's good approach : Any time you want to have a variable in a narrower scope than its surroundings, you can also use a freestanding block:

{
    let result;
    // ...
}

That would free you from using for without an increment expression (I'm of the school that all three parts of a for should be used, or use a different loop):

{
    let i = 0, result;
    while ((result = foo(i++)) < N) {
        // Do stuff...
    }
}

Live Example:

 const foo = x => x; const N = 5; { let i = 0, result; while ((result = foo(i++)) < N) { // Do stuff... console.log(result); } }