javscript 的默认参数

Posted 2020-10-22 liangcheng11

第一种：变量

function example(a,b){
    var a = arguments[0] ? arguments[0] : 1;//设置参数a的默认值为1
    var b = arguments[1] ? arguments[1] : 2;//设置参数b的默认值为2
    return a+b;
}

或者

function example(){
    var a = arguments[0] ? arguments[0] : 1;//设置第一个参数的默认值为1
    var b = arguments[1] ? arguments[1] : 2;//设置第二个参数的默认值为2
    return a+b;
}

第二种：原型链

function fn(opts) {
    var DefaultConf = function() {};
    DefaultConf.prototype = {
        name: ‘lc‘,
        age: 18
    }
    defaultConf = new DefaultConf();
    for(var i in opts) {
        defaultConf[i] = opts[i]
    }
}

这种方法缺点是在函数内声明了函数，会引发一系列纠结问题。

第三种：jquery的$.extend()（浅拷贝）

示例：

function example(setting){
    var defaultSetting={
        name:‘貂蝉‘,
        age:‘30‘,
        sex:‘女‘,
        phone:‘13611876347‘,
        QQ:‘10086‘,
        birthday:‘1949.10.01‘ 
    };
    $.extend(defaultSetting,settings);
    var message=‘姓名：‘+defaultSetting.name
    +‘，性别：‘+defaultSetting.sex
    +‘，年龄：‘+defaultSetting.age
    +‘，电话：‘+defaultSetting.phone
    +‘，QQ：‘+defaultSetting.QQ
    +‘，生日：‘+defaultSetting.birthday
    +‘。‘;
    alert(message);
}

jquery的$.extend()的源码：

jQuery.extend = jQuery.fn.extend = function() {
    var options, name, src, copy, copyIsArray, clone,
        target = arguments[ 0 ] || {},
        i = 1,
        length = arguments.length,
        deep = false;

    // Handle a deep copy situation
    if ( typeof target === "boolean" ) {
        deep = target;

        // Skip the boolean and the target
        target = arguments[ i ] || {};
        i++;
    }

    // Handle case when target is a string or something (possible in deep copy)
    if ( typeof target !== "object" && !isFunction( target ) ) {
        target = {};
    }

    // Extend jQuery itself if only one argument is passed
    if ( i === length ) {
        target = this;
        i--;
    }

    for ( ; i < length; i++ ) {

        // Only deal with non-null/undefined values
        if ( ( options = arguments[ i ] ) != null ) {

            // Extend the base object
            for ( name in options ) {
                src = target[ name ];
                copy = options[ name ];

                // Prevent never-ending loop
                if ( target === copy ) {
                    continue;
                }

                // Recurse if we‘re merging plain objects or arrays
                if ( deep && copy && ( jQuery.isPlainObject( copy ) ||
                    ( copyIsArray = Array.isArray( copy ) ) ) ) {

                    if ( copyIsArray ) {
                        copyIsArray = false;
                        clone = src && Array.isArray( src ) ? src : [];

                    } else {
                        clone = src && jQuery.isPlainObject( src ) ? src : {};
                    }

                    // Never move original objects, clone them
                    target[ name ] = jQuery.extend( deep, clone, copy );

                // Don‘t bring in undefined values
                } else if ( copy !== undefined ) {
                    target[ name ] = copy;
                }
            }
        }
    }

    // Return the modified object
    return target;
};

第四种： es6的语法

1. 解构
2. Object.assign()