tsconfig .

initial project add relative dependencies.

测试Header，todoList，以及undoList组件，使用单元测试方法测试，单元测试方法主要适用于工具函数，函数及组件之间耦合不高的文件

Often while writing tests you have some setup work that needs to happen before tests run, and you have some finishing work that needs to happen after tests run. Jest provides helper functions to handle this.

.resolves、async/await、Error handling、.rejects

es6 新特性 主要包含“类、模块化、箭头函数、函数参数默认值、模板字符串、解构赋值、延展操作符、对象属性简写、Promise、let与const”

javascript 变量创建及初始化过程描述