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function pet(words) {
    this.words = words;

    console.log(this.words);
    console.log(this === global); // true
}

pet('...');

function Pet(words) {
    // this指的是函数拥有者，this只能在函数内部调用。
    this.words = words;
    this.speak = function() {
        console.log(this.words);
        console.log(this);
    }
}

// new会新创建一个对象用来替代global，让this指向这个对象
var cat = new Pet('Miao');

cat.speak();