To determine whether an array has a value:

Array.indexOf(val) > -1 //Existence (defect: first, it is not semantic enough. Its meaning is to find the first occurrence position of parameter value, so it is not intuitive to compare whether it is not equal to - 1. Second, it uses strict equality operator (====) to judge internally, which will lead to the misjudgment of NaN. )
Array.includes(val)

Determine whether a field exists in the object:

obj["key"] != undefined//(defect: if the key is defined, and the value of very 2 is undefined)
!("key" in obj)
obj.hasOwnProperty("key")
typeof item.editFlag === "undefined"

Judge whether it is empty

JavaScript itself does not judge whether a variable is a null value function, because variables may be of string, object, number, boolean and other types, with different types and different judgment methods. Therefore, a function is written in the article to determine whether the JS variable is null. If it is undefined, null, "', NaN, false, 0, [], {}, the null string, all return true, otherwise false

function isEmpty(v) {
    switch (typeof v) {
    case 'undefined':
        return true;
    case 'string':
        if (v.replace(/(^[ \t\n\r]*)|([ \t\n\r]*$)/g, '').length == 0) return true;
        break;
    case 'boolean':
        if (!v) return true;
        break;
    case 'number':
        if (0 === v || isNaN(v)) return true;
        break;
    case 'object':
        if (null === v || v.length === 0) return true;
        for (var i in v) {
            return false;
        }
        return true;
    }
    return false;
}

test

isEmpty()              //true
isEmpty([])            //true
isEmpty({})            //true
isEmpty(0)             //true
isEmpty(Number("abc")) //true
isEmpty("")            //true
isEmpty("   ")         //true
isEmpty(false)         //true
isEmpty(null)          //true
isEmpty(undefined)     //true
typeof
typeof undefined === 'undefined'
typeof null === 'object'

//Boolean
typeof true === 'boolean'

//Number
typeof 1 === 'number'

//String
typeof '111' === 'string'

//Array
typeof ['a','b'] === 'object'

//Object
typeof {a:1,b:2} === 'object'

Judge array method:

instanceof Array
objName.constructor==Array //(defect: creating Array in different iframe s does not share prototype; inheriting Array is also true, such as: b.prototype=new Array)
length，splice //Length cannot be enumerated (defect: object.propertyIsEnumerable('length '))
Array.isArray(objName)
obj.prototype.toString.call(objName)==='[object Array]'