每次在循环JavaScript中运行时，等待函数结束

Question

我正在尝试调用一个函数并等待其结束。我尝试使用一个使用promises很好（在一个电话的情况下）的示例。

function firstFunction(layer)
{
    // I want to make some requests to my server which i don't know how much time it'll take
    console.log(layer + ' in firstFunction');
    var deferred = $.Deferred();
    deferred.resolve();
    return deferred.promise();
}

function secondFunction()
{
    var layers=['layer1','layer2','layer3'];
    var promise = null;
    layers.forEach(function(layer,idx,array) {
        promise = firstFunction(layer);
        promise.then(function(result) { 
            if (idx === array.length - 1){ 
                console.log(layer ' is the last layer in secondFunction');
            }
            else{
                console.log(layer+' in secondFunction');
            }
        });
    });
}

这是我的期望：

layer1 in firstFunction
layer1 in secondFunction
layer2 in firstFunction
layer2 in secondFunction
layer3 in firstFunction
layer3 is the last layer in secondFunction

但是我得到这个：

layer1 in firstFunction
layer2 in firstFunction
layer3 in firstFunction
layer1 in secondFunction
layer2 in secondFunction
layer3 is the last layer in secondFunction

编辑：
我避免使用setTimeout，因为我担心我的函数所需要的时间将超过我给定的时间。