var stack = [];

// 定义 index 和 next
var index = 0;

function next(){
    var fn = stack[index];
    index = index + 1;
    if ( typeof fn == 'function' ){
         fn();
    }
}

function fn1(){
    console.log('第一个调用');
    next(); // stack 中每一个函数都必须调用`next`
}

stack.push(fn1);

function fn2() {
    setTimeout(function(){
        console.log('第二个被调用');
        next();
    }, 0)
}

stack.push(fn2, function(){
    console.log('第三个调用');
    next();
})

next();