javascript - 功能的冗余类型检查？

Question

在 Array.prototype.forEach js mdn 的 Polyfill 部分，您可以找到以下检查:

if (typeof callback !== 'function') {
    throw new TypeError(callback + ' is not a function');
}

在这种特定情况下，为什么还要检查功能？在我的测试中，直接调用该函数会产生完全相同的行为

var callback;
callback();
// Uncaught TypeError: callback is not a function

var callback = "";
callback();
// Uncaught TypeError: callback is not a function

var callback = 2;
callback();
// Uncaught TypeError: callback is not a function

var callback = [];
callback();
// Uncaught TypeError: callback is not a function

var callback = {};
callback();
// Uncaught TypeError: callback is not a function

实际上这种字符串连接有点糟糕，因为我们可能会看到 [object Object] is not a function":

function newCallback(val){
    if (typeof val !== 'function') {
        throw new TypeError(val+ ' is not a function');
    }
}
newCallback({});
//Uncaught TypeError: [object Object] is not a function

Answer 1

In this specific case, why bother checking for function?

如果数组为空，则不会尝试调用该函数，但它 should still fail .

[].forEach('foo')

当元素访问有副作用时，非空数组也有区别:

let foo = {
    get [0]() {
        alert(1);
    }
};

Array.prototype.forEach.call(foo, …);
// alert shows up if the early type check is missing