In what cases, we want to use apply with this?
I understand how this works but cant understand why we want to use apply with "this" keyword as in a example below:
function curry(func) {
return function curried(...args) {
if (args.length >= func.length) {
return func.apply(this, args)
} else {
return curried.bind(this, ...args)
}
}
}
Here bind and apply use "this" as a first argument, but what is the purpose if we can just do func(args) since this points to same lexical environment of a function. I can see some benefits with it with arrow functions but here I have named functions. There is no difference or I am missing something?
The reason for using apply is to keep the same the value of this. Calling func(args) would result in this being the window object (in non-strict mode) or undefined (in strict mode).
Here's an example that breaks if you call func(args):
function curry(func) {
return function curried(...args) {
if (args.length >= func.length) {
return func(args);
} else {
return curried.bind(this, ...args);
}
};
}
const example = {
multiplier: 5,
calculate: function (a, b) {
return (a + b) * this.multiplier;
},
};
example.curriedVersion = curry(example.calculate);
// Because of the way these are being called, `this` should be `example`
console.log(example.calculate(1, 2));
// But here it's not, causing unexpected results
console.log(example.curriedVersion(1)(2));But it works if you do apply(this, args):
function curry(func) {
return function curried(...args) {
if (args.length >= func.length) {
return func.apply(this, args);
} else {
return curried.bind(this, ...args);
}
};
}
const example = {
multiplier: 5,
calculate: function (a, b) {
return (a + b) * this.multiplier;
},
};
example.curriedVersion = curry(example.calculate);
// Because of the way these are being called, `this` should be `example`
console.log(example.calculate(1, 2));
console.log(example.curriedVersion(1)(2));