SoFunction
Updated on 2025-04-03

The difference between the use and difference between the toString function and the valueOf function in js

1. 👀 Preface

In the equality operator, if the content of the comparison contains object type data, implicit conversion will be involved, and the toString() function and valueOf() function will be called. Below we will learn about the basic concepts and usage scenarios of these two functions.

2. toString() function

The function toString() function is to convert a logical value into a string and return the result.

((1).toString());//1
((10).toString(2))//1010
(("1").toString());//1
((false).toString());//false
(({p:1}).toString());//[object,object]
((undefined).toString());//Report an error((null).toString());//Report an error((function(){}).toString());//function(){}
([1,2,3,4].toString());//1,2,3,4
((new Date()).toString())//Fri Jul 03 2020 17:20:11 GMT+0800 (China Standard Time)

In JavaScript, Object, Array, Function, Date and other types all implement custom toString() functions.

  • The default return result of the toString() function of Object type data is "[object Object]". When we customize a new class, we can override the toString() function to return more readable results.
  • The toString() function of Array returns a comma-separated array member string. Arrays are objects, so why does the array return the corresponding string instead of the object? In fact, the array overwrites the method, and then joins the array and returns a string containing each array element separated by a comma (equivalent to())
  • Function's toString() function returns the value of the function's text definition
  • The toString() function of Date returns a time string with readability

3. valueOf() function

The function of the valueOf() function is to return the original value that is most suitable for the reference type. If there is no original value, it will return the reference type itself.

((1).valueOf());//1
((10).valueOf(2))//Report an error(("1").valueOf());//1
((false).valueOf());//false
(({p:1}).valueOf());//{p:1}
((undefined).valueOf());//Report an error((null).valueOf());//Report an error((function(){}).valueOf());//function(){}
([1,2,3,4].valueOf());//[1,2,3,4]
((new Date()).valueOf());//1593767848260
  • The default return result of the valueOf() function of Object type data is "{}", that is, an empty object literal.
  • Array's valueOf() function returns the array itself
  • The valueOf() function of function returns the function itself
  • The valueOf() function of Date returns a time stamp of the specified date.

summary:

  • toString() returns a string, while valueOf() returns the original value, without the original value, the object itself
  • Neither undefined nor null have toString() and valueOf() methods
  • Date type toString() returns a string representing time; valueOf() returns the number of milliseconds (timestamp) from now to January 1, 1970
  • The toString() method of the Number type can receive the conversion cardinality and return the value in the form of a string in different digits; while the valueOf() method cannot accept the conversion cardinality

4. Special circumstances

If a reference type's value exists both toString() and valueOf() functions, which function will be called when doing implicit conversion? Here we can summarize it into two scenarios, namely, converting the reference type to String type, and converting the reference type to Number type.

1. Convert the reference type to String type

When a reference type is converted to String type, it is generally used for data display. The following rules are followed when converting:

  • If the object has the toString() function, the toString() function is called first. If it returns a primitive value, the original value will be converted directly into a string representation and the string will be returned.
  • On the contrary, the valueOf() function will be called again. If the result returned by the valueOf() function is a primitive value, the result will be converted into a string representation and the string will be returned.
  • If a primitive value cannot be obtained through the toString() function or the valueOf() function, a type conversion exception will be directly thrown.

2. Convert the reference type to the Number type

When a reference type is converted to Number type, it is generally used for data calculation. The following rules are followed when converting:

  • If the object has a valueOf() function, the valueOf() function will be called first, and if the valueOf() function returns a primitive value, the original value will be converted directly into a numeric representation and returned the number.
  • On the contrary, the toString() function will be called again. If the result returned by the toString() function is an original value, the result will be converted into a numeric representation and the number will be returned.
  • If a primitive value cannot be obtained through the toString() function or the valueOf() function, a type conversion exception will be directly thrown.

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