js 实现call

手写call

/**
 * 实现call
 * @param {*} context 
 * fn.call(target,...args)
 * 让fn中的this指向target
 * 思路是函数默认指向调用者，在目标对象上挂载fn，执行的时候fn中的this默认指向目标对象， 执行完拿到结果并删除这个挂载属性即可
 */
Function.prototype._call = function(context){
    if(typeof this !== 'function'){
        throw 'Error'
    }

    context = context || window
    context.fn = this
    const args = Array.from(arguments).slice(1)
    const result = context.fn(...args)
    delete context.fn
    return result    
}

const fn = function(){console.log(this); return {a:1,...this}}
fn._call({test:1})