将参数传递给Array.forEach回调函数

Question

someOperation.then(function(x) {
    things.forEach(function(thing) {
        //doing something with 'thing' that depends on variable 'x'
    });
});

In the code above, how can I make the variable 'x' available inside the callback function? Or do I have to go back to using a for loop in this case?

Answer 1

You can pass a "thisArg" as the second parameter to forEach so for instance:

let x = { a: 123 };
things = ['foo', 'bar']
things.forEach(function(thing) {
    alert( this.a + thing );
}, x);

Might be helpful depending on what you are trying to do.

Answer 2

It is available.

 let x = { name: 'Mike' }; ['Hello', 'Goodbye'].forEach(function(greeting) { document.querySelector('pre').innerHTML += greeting + ', ' + x.name + '\\n'; }); 

 <pre></pre> 

What you're using here is known as a closure and is a commonly used feature of Javascript. Basically, any function has access to any other variable in it's parent scope.

 function log(msg) { document.querySelector('pre').innerHTML += msg + '\\n'; } var global = 'global'; function firstLevel(third) { var first = 'first'; // `global` is in the parent scope so we can access it log(global + ' -> ' + first); function secondLevel() { var second = 'second'; // Same thing with `first` here log(global + ' -> ' + first + ' -> ' + second); // This even works with passed in arguments log(global + ' -> ' + first + ' -> ' + second + ' -> ' + third); // We can even change closed over variables first = 'fourth?'; } secondLevel(); log(first); // Notice that `first` changed. } log(global); firstLevel('third'); // Notice how `third` is used in `secondLevel` 

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