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  • Type checking

    // Check to see if our number is actually a string

    if ( typeof num == "string" )

    // If it is, then parse a number out of it

    num = parseInt( num );

    // Check to see if our array is actually a string

    if ( typeof arr == "string" )

    // If that's the case, make an array, splitting on commas

    arr = arr.split(",");



    The use of the typeof statement helps to lead us into the topic of type-checking. Since

    JavaScript is (currently) a dynamically typed language, this proves to be a very useful and

    important topic. There are a number of different ways to check the type of a variable; we’re

    going to look at two that are particularly useful.

    The first way of checking the type of an object is by using the obvious-sounding typeof

    operator. This utility gives us a string name representing the type of the contents of a variable.

    This would be the perfect solution except that for variables of type object or array, or a custom

    object such as user, it only returns object, making it hard to differentiate between all objects.

    The second way of checking the type of an object is by referencing a property of all

    JavaScript objects called constructor. This property is a reference to the function used to

    originally construct this object.

    // Check to see if our number is actually a string

    if ( num.constructor == String )

    // If it is, then parse a number out of it

    num = parseInt( num );

    // Check to see if our string is actually an array

    if ( str.constructor == Array )

    // If that's the case, make a string by joining the array using commas

    str = str.join(',');



    Table 2-1 shows the results of type-checking different object types using the two differ

    methods that I’ve described. The first column in the table shows the object that we’re tryin

    find the type of. The second column is the result of running typeof Variable (where Variabl

    the value contained in the first column). The result of everything in this column is a string.

    Finally, the third column shows the result of running Variable.constructor against the objec

    contained in the first column. The result of everything in this column is an object.

    Table 2-1. Type-Checking Variables

    Variable typeof    Variable     Variable.constructor

    { an: “object” }   object       Object

    [ “an”, “array” ]  object       Array

    function(){}       function     Function

    “a string”         string       String

    55                 number       Number

    true               boolean      Boolean

    new User()         object       User

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  • 原文地址:https://www.cnblogs.com/coolicer/p/2215137.html
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