In JavaScript, getting "a == 1 && a == 2 && a == 3" to hold sounds like an impossible task. After all, a is either equal to one of 1, 2, 3, or not equal to any of them. However, there is a strange trick that makes this condition true. In this post, we'll explore this trick and delve into the rationale behind it.
First, let's see what the condition "a == 1 && a == 2 && a == 3" means. what does it mean It means that a must be equal to 1, 2 and 3. However, this is not possible because a can only have one value. So, we need a way to trick JavaScript into thinking a is actually equal to 1, 2, and 3. Here's one way to achieve this:
var a = {
i: 1,
toString: function() {
return a.i++;
}
};
if (a == 1 && a == 2 && a == 3) {
console.log("条件成立！");
}
This code looks weird. We define an object called a that has a property i and a toString method. The initial value of i is 1, the toString method returns the value of i, and adds 1 to the value of i. So, when we use a in the condition, JavaScript calls the a.toString() method, which returns the value of i and increments the value of i by 1. This means we can trick JavaScript into thinking a is equal to 1, 2, and 3 by modifying the value of i.
Let's explain the process step by step. First, JavaScript tries to convert a to a boolean when we use it in the condition. Because a is an object, JavaScript calls the a.valueOf() method, which returns the object itself, since objects are truthy. JavaScript then calls the a.toString() method, which returns the value of i, which is 1. So, we now have an equation:
if (1 == 1 && a == 2 && a == 3) {
console.log("条件成立！");
}
Next, JavaScript calls the a.toString() method again, which returns the value of i, which is 2. Now, we have an equation:
if (1 == 1 && 2 == 2 && a == 3) {
console.log("条件成立！");
}
Finally, JavaScript calls the a.toString() method again, which returns the value of i, which is 3. Now, we have an equation:
if (1 == 1 && 2 == 2 && 3 == 3) {
console.log("条件成立！");
}
This equation is true, so the condition holds! We have successfully tricked JavaScript into thinking that a is equal to 1, 2, and 3.
But what's the rationale behind this trick? Why can we change the value of an object by modifying the toString method? This involves the mechanism of type conversion and object property access in JavaScript.
First, let's look at type conversions in JavaScript. In JavaScript, there are two types of conversions: explicit conversions and implicit conversions. Explicit conversions are performed by calling type conversion functions such as Number(), String(), and Boolean(). Implicit conversions happen automatically when a value of a different type is required, for example when adding a string to a number, JavaScript converts the string to a number.
Implicit conversions play a key role in our example. When we compare a to the numbers 1, 2, and 3, JavaScript will call the a.valueOf() and a.toString() methods, which is a result of the implicit conversion happening.
Second, let's look at the mechanics of object property access. In JavaScript, there are two ways to access object properties: dot notation and square bracket notation. Dot notation is to access properties through the dot between the object name and the property name, such as obj.property. The square bracket notation is to access properties through the square brackets between the object name and the property name, such as obj['property']. The two methods behave slightly differently when accessing properties.
Dot notation requires the property name to be a valid identifier, i.e. it cannot contain spaces or operators. If the property name is not a valid identifier, square bracket notation must be used. Bracket notation can use any string as a property name, including strings containing spaces and operators.
Another important difference is that square bracket notation can use variables as property names. For example, if we have a variable name, we can use obj[name] to access properties of the object, something that dot notation cannot do.
In our case, we used an object to simulate the value of a. This object has a property called i with a value of 1. We also define a toString method for the object, which returns the value of i and increments the value of i by 1. Because we use the square bracket notation to access the properties of the object, we can use the variable i as the property name, which allows us to change the value of the property of the object by modifying the value of i.
To sum up, we have successfully used the type conversion and object property access mechanism in JavaScript to trick JavaScript into thinking that a is equal to 1, 2, and 3. Weird as this trick may seem, it demonstrates some interesting and powerful mechanisms in JavaScript. When writing JavaScript code, understanding these mechanisms can help us better grasp the language.
So, how do we apply these mechanisms to our problem? Here is a code that implements this problem:
const a = {
i: 1,
toString: function () {
return this.i++;
}
};
if (a == 1 && a == 2 && a == 3) {
console.log('a == 1 && a == 2 && a == 3');
}
In this code, we define an object a that has a property called i with an initial value of 1. We also define a toString method for this object, which increments the value of i and returns the old value of i. This is how we leveraged the object property access mechanism we discussed earlier.
Then, we compare whether a is equal to 1, 2, and 3 in the if statement. In this comparison, JavaScript will convert a to a string type. Since a is an object, JavaScript calls a's toString method to perform the conversion. Each time the toString method is called, the value of i is incremented by 1, so the first call returns 1, the second call returns 2, and the third call returns 3.
When a is compared to the numbers 1, 2, and 3, they are all string types, so JavaScript converts them to number types. Since the strings "1", "2" and "3" can all be converted to the numbers 1, 2 and 3, the comparison evaluates to true and the code block of the if statement is executed.
This is how we trick JavaScript into making a equal to 1, 2, and 3 using the type conversion and object property access mechanism. Of course, this is not good programming practice. This technique tends to make the code difficult to understand and maintain, and may produce different results in different JavaScript engines. When writing real applications, we should avoid this trick and use cleaner and easier to understand code.