TypeScript/JavaScript forEach call

Question

I am having trouble understanding this bit of code:

stringsArray.forEach(s => {
    for (var name in validators) {
        console.log('"' + s + '" ' +
            (validators[name].isAcceptable(s) ?
                ' matches ' : ' doesnt match ') + name);
    }
});

in particular, the s => { ... part is mysterious. It looks like s is being assigned to the next string in the array on each loop. But what is the => part meaning? It's related to lambdas I think, but I am not following.

Josh Beam · Accepted Answer · 2015-06-20 01:41:07Z

16

Yeah it's a lambda (for example, similar to ECMAScript6 and Ruby, as well as some other languages.)

Array.prototype.forEach takes three arguments, element, index, array, so s is just the parameter name being used for element.

It'd be like writing this in regular ECMAScript5:

stringsArray.forEach(function(s) {
    for (var name in validators) {
        console.log('"' + s + '" ' +
            (validators[name].isAcceptable(s) ?
                ' matches ' : ' doesnt match ') + name);
    }
});

In the above example, you didn't show the whole code, so I assume validators is just a plain object {}.

The syntax for the example you gave is actually identical to the ES6 syntax.

Check out this example from the TypeScript handbook:

example

answered Jun 20, 2015 at 1:41

Josh Beam

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3 Comments

pitosalas Over a year ago

Thanks... Still a little confused... Note that I am quite proficient in Ruby but not in JS. I am learning TS instead because it seems like a more solid foundation for me... is "s => { }" passing a lambda as the first argument to for_each? If so, how can s appear again in the body of the lambda itself? That's where I am stumped (for now.) Thanks!

Josh Beam Over a year ago

@pitosalas, TypeScript is icing on top of JavaScript. I think it's probably a bad idea to learn TypeScript first, and not the other way around. Have you read the documentation for .forEach? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

pitosalas Over a year ago

well I know a little js (and I am reading eloquent js in parallel) so other than some trickiness here I feel pretty comfortable. But I will follow the link you point to and study that too. Thanks.

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