一、背景

最近加入了一個刻意練習(xí)小組谒主，自選了一個課題完域。
題目：《實現(xiàn)一個前端異常收集器》
目標：收集前端的各類錯誤，包括收集時間瘩将、容錯等吟税。

先介紹一下思路：

思路

二、github源碼

安裝：
yarn add web-error-tracker

https://github.com/evilrescuer/web-error-tracker

測試項目：showcase

三姿现、常見前端異常類型

此處為示例片段代碼肠仪，具體請查看github源碼

1.JavaScript語法異常

如Uncaught ReferenceError: t is not defined

function testJavaScriptSyntaxError() {
    t();
}

testJavaScriptSyntaxError();

2.加載圖片資源異常

加載圖片錯誤

function testImgError() {
    const img = document.createElement('IMG');
    img.src = './test.png';
    document.body.append(img);
}

testImgError();

3.未捕獲的Promise錯誤

沒有捕獲 unhandledrejection錯誤

function testPromiseError() {
    new Promise((resolve, reject) => {
        t();
    });
}

testPromiseError();

4.api返回錯誤

// Node.js啟動一個server，模擬接口返回500
if (ctx.req.url === '/test500') {
    ctx.body = 'Internal Server Error';
    ctx.status = 500;
}

// 測試
// 事先引入axios庫
// ...
function testCallApiError() {
    axios.get('http://localhost:3000/test500');
}

testCallApiError();

5.跨域異常

// html
<button id="btn-cors-error">跨域異常</button>
<script src="http://localhost:3000/file.js" crossorigin></script>

// Node.js服務(wù)模擬file.js返回
if (ctx.req.url === '/file.js') {
    ctx.set('Content-Type', 'text/javascript');
    ctx.body = `
        const btn = document.querySelector('#btn-cors-error');
        btn.addEventListener('click', () => {
            // b is not defined
            var a = b;
        });
    `;
}

注：必須加上crossorigin备典，否則捕獲的錯誤不夠詳細异旧，而是Script Error

6.動態(tài)創(chuàng)建的有錯誤的腳本

function testCreateAWrongScriptError() {
    const script = document.createElement('script');
    // testCreateAWrongScriptErrorValiable is not defined
    script.innerHTML = ` 
        var a = testCreateAWrongScriptErrorValiable;
    `;
    document.body.append(script);
}

testCreateAWrongScriptError()

7.iframe內(nèi)部異常

// html
<iframe src="./iframe.html" frameborder="0"></iframe>

// iframe
<script>
    setTimeout(() => {
        // b is not defined
        var a = b;
    }, 1000)
</script>

四、源碼解析

// 修改原生EventTarget對象
function modifyEventTarget (destWindow) {
    // 跨域異常-crossOrigin
    const originAddEventListener = destWindow.EventTarget.prototype.addEventListener;
    destWindow.EventTarget.prototype.addEventListener = function (type, listener, options) {
        const wrappedListener = function (...args) {
            try {
                return listener.apply(this, args);
            }
            catch (err) {
                throw err;
            }
        };
        return originAddEventListener.call(destWindow, type, wrappedListener, options);
    };
}

// 修改原生XMLHttpRequest
function hookAjax (proxy, destWindow = window) {
    const realXhr = "RealXMLHttpRequest";
    destWindow[realXhr] = destWindow[realXhr] || destWindow.XMLHttpRequest;

    destWindow.XMLHttpRequest = function () {
        const xhr = new destWindow[realXhr];
        for (const attr in xhr) {
            let type = "";
            try {
                type = typeof xhr[attr];
            } catch (e) {
            }
            if (type === "function") {
                this[attr] = hookFunction(attr);
            } else {
                Object.defineProperty(this, attr, {
                    get: getterFactory(attr),
                    set: setterFactory(attr),
                    enumerable: true
                });
            }
        }
        this.xhr = xhr;

    };

    function getterFactory(attr) {
        return function () {
            const v = this.hasOwnProperty(attr + "_") ? this[attr + "_"] : this.xhr[attr];
            const attrGetterHook = (proxy[attr] || {})["getter"];
            return attrGetterHook && attrGetterHook(v, this) || v
        }
    }

    function setterFactory(attr) {
        return function (v) {
            const xhr = this.xhr;
            const that = this;
            const hook = proxy[attr];
            if (typeof hook === "function") {
                xhr[attr] = function () {
                    proxy[attr](that) || v.apply(xhr, arguments);
                }
            } else {
                const attrSetterHook = (hook || {})["setter"];
                v = attrSetterHook && attrSetterHook(v, that) || v
                try {
                    xhr[attr] = v;
                } catch (e) {
                    this[attr + "_"] = v;
                }
            }
        }
    }

    function hookFunction(fun) {
        return function () {
            const args = [].slice.call(arguments);
            if (proxy[fun] && proxy[fun].call(this, args, this.xhr)) {
                return;
            }
            return this.xhr[fun].apply(this.xhr, args);
        }
    }

    return destWindow[realXhr];
}

class ErrorTracker {
    constructor() {
        this.errorBox = [];
    }

    init() {
        this.handleWindow(window);
    }

    getErrors() {
        return this.errorBox;
    }

    handleWindow(destWindow) {
        const _instance = this;
        modifyEventTarget(destWindow);

        // XHR錯誤（利用http status code判斷）
        hookAjax({
            onreadystatechange: xhr => {
            if (xhr.readyState === 4) {
                if (xhr.status >= 400 || xhr.status <= 599) {
                    console.log('xhr錯誤：', xhr);
                    const error = xhr.xhr;
                    _instance.errorBox.push(new FEError(`api response ${error.status}`, error.responseURL, null, null, error.responseText));
                }
            }
        }
        }, destWindow);

        // 全局JS異常-window.onerror / 全局靜態(tài)資源異常-window.addEventListener
        destWindow.addEventListener('error', event => {
            event.preventDefault();
            console.log('errorEvent錯誤:', event);
            if (event instanceof destWindow.ErrorEvent) {
                _instance.errorBox.push(new FEError(event.message, event.filename, event.lineno, event.colno, event.error));
            }
            else if (event instanceof destWindow.Event) {
                if (event.target instanceof HTMLImageElement) {
                    _instance.errorBox.push(new FEError('load img error', event.target.src, null, null, null));
                }
            }
            return true;
        }, true);

        // 沒有catch等promise異常-unhandledrejection
        destWindow.addEventListener('unhandledrejection', event => {
            event.preventDefault();
            console.log('unhandledrejection錯誤:', event);
            _instance.errorBox.push(new FEError('unhandled rejection', null, null, null, event.reason));
            return true;
        });

        // 頁面嵌套錯誤（iframe錯誤等等提佣、單點登錄）（注意：不能捕獲iframe加載時的錯誤）
        destWindow.addEventListener('load', () => {
            const iframes = destWindow.document.querySelectorAll('iframe');
            iframes.forEach(iframe => {
                _instance.handleWindow(iframe.contentWindow);
            });
        });
    }
}

class FEError {
    constructor(message, source, lineno, colno, stack) {
        this.message = message;
        this.source = source;
        this.lineno = lineno;
        this.colno = colno;
        this.stack = stack;
        this.time = new Date();
    }
}

window.errorTracker = new ErrorTracker();
window.errorTracker.init();

五吮蛹、總結(jié)

總體思路：通過監(jiān)聽error事件、修改原生xhr拌屏、修改EventTarget潮针。
如有iframe，需要在iframe的window環(huán)境下倚喂，遞歸以上處理每篷。

(完)