Question

试试playground：

typeof a == "object"

将导致

运算符'=='无法应用于类型'“string”| “数字”| “符号”| “对象”| “function”'和'“array”'。）

这个错误消息不仅对我毫无意义（a instanceof Array也没关系），它似乎也是最近的变化。您现在必须使用<div id="jsGrid"></div> <script> var id = "<?php echo $id?>" //the id being passed to the View. var resources = []; var instruments = [ {Name: "", Id: 0}, {Name: "Drum Kit", Id: 1}, {Name: "Guitar", Id: 2}, {Name: "Piano", Id: 3}, {Name: "Violin", Id: 4}, ]; $("#jsGrid").jsGrid({ controller: { loadData: function (filter) { $.ajax({ type: "GET", url: "<?= $host.$basepath ?>/instrument/", data: filter }); }, insertItem: function (item) { $.ajax({ type: "POST", url: "<?= $host.$basepath ?>/instrument/", data: item }); }, updateItem: function (item) { $.ajax({ type: "PUT", url: "<?= $host.$basepath ?>/instrument/", data: item }); }, deleteItem: function (item) { $.ajax({ type: "DELETE", url: "<?= $host.$basepath ?>/instrument/", data: item }); } }, width: "100%", height: "400px", inserting: true, editing: true, sorting: true, paging: true, data: resources, fields: [ { name: "Instrument", type: "select", items: instruments, valueField: "Id", textField: "Name", validate: "required", width: 175 }, {name: "Comment", type: "text", width: 150}, {name: "resource_id", type: "text", width: 150, visible: false, insertTemplate: function() { var input = this.__proto__.insertTemplate.call(this); input.val(id) return input; }}, {type: "control"} ] }); </script> </div>，这是我上次检查时没有必要的。你能解释一下吗？

Answer 1

正如SLaks所指出的，typeof运算符永远不会返回＆＃34; array＆＃34;。 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

它从来没有奏效，但TypeScript只是变得足够聪明，可以在2.0中警告你。

Answer 2

以下是一个完整的工作示例：

var a: string | string[];

// You need this in the example to ensure the compiler won't determine that a
// is never a string if you set it to an array...
if (new Date().getDay() > 3) {
    a = 'a string';
} else {
    a = [
        'an',
        'array'
    ];
}

if (Array.isArray(a)) {
    var example1 = a; // example1: string[]
} else {
    var example2 = a; // example2: string
}

中间代码块仅用于演示目的，如果以var a: string | string[] = []开始此示例，变量example2实际上将最终为never类型，因为编译器足够智能看到没有a变量将被设置为除数组之外的任何内容的情况。

typeof a ==“array”有什么问题？

2 个答案: