关于 JavaScript 数组原型

Question

I have recently read tutorial associated with JavaScript from this page , I tried some piece of code but i didn't understand some logic. Below is author code.

function itself:

var lz="";
lz.memo = function (fn) {
var cache = {};
return function () {
    var key = [].join.call(arguments, '§') + '§';
    if (key in cache) {
        return cache[key];        }
    return cache[key] = fn.apply(this, arguments);
  };
};

and its execution

 var foo = 1;
function bar(baz) {
    return baz + foo;
}
var cached = lz.memo(bar);
console.log(cached(1));
foo += 1;
console.log(cached(1)); //2

But if I change

var key = [].join.call(arguments, '§') + '§';

to

var key=arguments[0];

it also works (Caching works). What is the purpose here the author used

var key = [].join.call(arguments, '§') + '§';

Thank for attention! Here is CODEPEN of code

Answer 1

it also works (Caching works)

Only because you called it with just one argument. (And it didn't quite work the same way, the cache key will be missing the § on the key that the original code had.)

What is the purpose here the author used

They're creating a string with all of the arguments you supply joined together with § between them and § at the end, like this:

 function foo() { return [].join.call(arguments, '§') + '§'; } console.log(foo(1)); // 1§ console.log(foo(1, 2)); // 1§2§ console.log(foo("a", "b", "c")); // a§b§c§

They're doing it because arguments is array-like , but it isn't an array, and so it doesn't have the join method from Array.prototype .

There's actually no reason for the temporary array they create, you can just call join directly:

return Array.prototype.join.call(arguments, '§') + '§';

Another opton is to convert arguments to an actual array:

return Array.prototype.slice.call(arguments).join('§') + '§';

Or in ES2015 (or with a shim):

return Array.from(arguments).join('§') + '§';