Introduction to the rules for converting objects into numbers or strings in JavaScript_javascript skills

During the writing process of JavaScript programs, JS will automatically convert the object into a number or string according to different contexts before processing. The rules for this automatic conversion are as follows:

Rule for automatic conversion of object into string:

1. If the class to which the object belongs overrides the toString() method, call this method. If the return result of the toString() call is Primitive (string, number, boolean, undefined, null), the Primitive value is converted into a string and returned.

2. If the class to which the object belongs does not override the toString() method - the return result of the toString() call is "[object Object]"; or if the toString() method is overridden but the method returns an object. Then JS will call the valueOf() method of object. If the return result of the valueOf() call is Primitive (string, number, boolean, undefined, null), the Primitive value will be converted into a string and returned.

3. If neither of the above two points are met and the Primitive value cannot be obtained by calling the object's toString() method or valueOf() method, then JS will throw a TypeError.

Rule for automatic conversion of object into number:

1. Call the valueOf() method of object. If the value obtained is a Primitive value, convert the Primitive value into a number and return it.

2. If the Primitive value cannot be obtained from the valueOf() method, then call the object's toString() method; if toString() returns a Primitive value, convert the Primitive value into a number and return it.

3. If neither of the above two points are met, JS will throw a TypeError.

As you can see, the rules for automatic conversion of object into string and automatic conversion of object into number are actually the same. The difference lies in the calling order of toString() method and valueOf() method.

According to the above rules, some conversion results can be well understood:

1. For an empty array, when converting it to a number, the result is 0. This is because the valueOf() method of array will be called first. Since valueOf() returns the array object itself, JS will then call the toString() method of the empty array; because the return result of toString() of the empty array is a null character. String, so the empty string will eventually be converted into the number 0 and returned.

2. For an array with only one numeric member, apply the same rules to convert it into a number, and the final result will be the number.

3. For arrays with multiple numeric members, since the string cannot be converted into number, the final result is NaN.

When is it converted to string? When is it converted to number?

When performing automatic type conversion on an object, JS will choose to convert it into a string or a number depending on the object type and the operator. The specific rules are as follows:

1. If an object appears on both sides of the operator, convert the object into a string.
2. All objects (except Date objects) are converted to numbers first.
3. For Date objects, convert them to string first.

It is worth noting that for operators, except when objects or strings appear on both sides of the operator, the "convert to number" operation is performed in all other cases. At the same time, this operation is also related to the order between values. .

Experiment

Copy code The code is as follows:

console.log(3 * []);//0
console.log(3 * [2]);//6
console.log(3 * [1,2]);//NaN

console.log(3 [2]);//32

var now = new Date();
console.log(now 1);//Wed Mar 26 2014 10:51:39 GMT 0800 (CST)1
console.log(now - 1);//1395802299223
console.log(now * 2);//2791604598448

console.log(true true);//2
console.log(2 null);//2, null is converted to 0
console.log(2 undefined);//NaN, undefined is converted to NaN
console.log(1 2 "cats");//3 cats
console.log(1 (2 "cats"));//12 cats