Judgment type in JS

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There are generally four methods to judge the type in JS
1,
typeof: As everyone knows about this method, there are many types that cannot be determined, and it is not recommended.
2,
constructor: Each function will automatically have a prototype attribute. This property is an object, and this object contains the only non-enumerable property constructor. The value of the constructor attribute is a function object, and this function is the constructor;

function Aaa(){};
var a1 = new Aaa();

a1.constructor points to It is Aaa(); this constructor

So we can use it to do type judgment

var arr = [];
arr.constructor == Array This way we can judge whether it is Array, the same is true for other types

三
instanceof: Determine whether an object and the constructor are on a prototype chain;

function Aaa(){};
var o = new Aaa();

o instanceof Array : Check whether o and Array are on the same prototype chain. Obviously o is an instance under the Aaa constructor, so we get false;

o instanceof obj.prototype gets true; because all objects inherit obj.prototype

4,
Use the toString() method: Object.prototype.toString.call(o);
o : is an object instance. The above will return a string of type '[Object Array]'. You can judge the data type based on this string.

Summary, the fourth general method is the most reliable and safe. , Methods two and three generally have no problems, but they will go wrong when there is an iframe. One is the most unreliable

There are generally four methods to determine the type in JS
One,
typeof: Everyone knows this method. There are many types that cannot be determined. It is not recommended.
2.
constructor: Each function will automatically have a prototype attribute. This property is an object, and this object contains the only non-enumerable property constructor. The value of the constructor attribute is a function object, and this function is the constructor;

function Aaa(){};
var a1 = new Aaa();

a1.constructor points to It is Aaa(); this constructor

So we can use it to do type judgment

var arr = [];
arr.constructor == Array This way we can judge whether it is Array, the same is true for other types

三
instanceof: Determine whether an object and the constructor are on a prototype chain;

function Aaa(){};
var o = new Aaa();

o instanceof Array : Check whether o and Array are on the same prototype chain. Obviously o is an instance under the Aaa constructor, so we get false;

o instanceof obj.prototype gets true; because all objects inherit obj.prototype

4,
Use the toString() method: Object.prototype.toString.call(o);
o : is an object instance. The above will return a string of type '[Object Array]'. You can judge the data type based on this string.

Summary, the fourth general method is the most reliable and safe. , the second and third methods generally have no problems, but they will go wrong when there is an iframe. The first is the most unreliable

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