奇怪的`方法不能在可能的null/undefined值`上调用

Question

以下缩小的代码:

// @flow'use strict';

import assert from 'assert';

class Node<V, E> {
    value: V;
    children: ?Map<E, Node<V,E>>;

    constructor(value: V) {
        this.value = value;
        this.children = null;
    }
}


function accessChildren(tree: Node<number, string>): void {

    if (tree.children!=null) {
        assert(true); // if you comment this line Flow is ok
        tree.children.forEach( (v,k)=>{});
    } else {
    }

}

...使用以下消息进行流类型检查:

$ npm run flow

> simple-babel-serverside-node-only-archetype@1.0.0 flow /home/blah/blah/blah
> flow; test $? -eq 0 -o $? -eq 2

es6/foo.js:21
21:             tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call of method `forEach`. Method cannot be called on possibly null value
21:             tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^ null

es6/foo.js:21
21:             tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call of method `forEach`. Method cannot be called on possibly undefined value
21:             tree.children.forEach( (v,k)=>{});
^^^^^^^^^^^^^ undefined


Found 2 errors

如果行读数:assert(true)已注释掉,则流程满足!

是什么赋予了？

PS:如果有人想知道,我.flowconfig,.babelrc和package.json文件是不伦不类:

.flowconfig

$ cat .flowconfig
[options]
esproposal.class_static_fields=enable

.babelrc

$ cat .babelrc
{
"presets": ["es2015"],
"plugins": ["transform-object-rest-spread", "transform-flow-strip-types", "transform-class-properties"]
}

的package.json

$ cat package.json
{
"name": "simple-babel-serverside-node-only-archetype",
"version": "1.0.0",
"description": "",
"main": [
"index.js"
],
"scripts": {
"build": "babel es6 --out-dir es5 --source-maps",
"build-watch": "babel es6 --out-dir es5 --source-maps --watch",
"start": "node es5/index.js",
"flow": "flow; test $? -eq 0 -o $? -eq 2"
},
"author": "",
"license": "ISC",
"devDependencies": {
"babel-cli": "^6.6.5",
"babel-core": "^6.7.4",
"babel-plugin-transform-class-properties": "^6.10.2",
"babel-plugin-transform-flow-strip-types": "^6.8.0",
"babel-polyfill": "^6.7.4",
"babel-preset-es2015": "^6.9.0",
"babel-runtime": "^6.6.1",
"flow-bin": "^0.27.0"
},
"dependencies": {
"babel-plugin-transform-object-rest-spread": "^6.8.0",
"babel-polyfill": "^6.7.4",
"source-map-support": "^0.4.0"
}
}

Answer 1

此处描述了您的情况。

\n\n

Flow 无法知道，这assert不会改变tree。\n将以下行添加到您的代码中并运行它 \xe2\x80\x93 您将遇到运行时错误，因为断言函数将在调用时设置tree.children为null。

\n\n

const root = new Node(1);\nconst child = new Node(2);\n\nroot.children = new Map([[\'child\', child]]);\n\nassert = () => root.children = null;\n\naccessChildren(root);\n

\n\n

是的，这是相当奇怪的代码，但 Flow 不知道，你也不会写它。

\n