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The more javascript code is produced in SharePoint solutions, the more need we have to log information and possible errors to a central logging  place in SharePoint: ULS. This blog post is about logging to ULS from javascript.

For a while ago I read a blog post:

• 5 suggestions to implement a better logging in SharePoint

The author @avishnyakov mentions the ability log to ULS from javascript. I want to dive deeper.

ULS.enable = true
ULSOnError("Hello from javascript", location.href, 0);

What this function actually does, is that it calls a web service called _vti_bin/diagnostics.asmx

We can follow the function in the init.debug.js

function ULSOnError(msg, url, line) {
    return ULSSendExceptionImpl(msg, url, line, ULSOnError.caller);
}

ULSOnError invokes ULSSendExceptionImpl:

function ULSSendExceptionImpl(msg, url, line, oCaller) {
    if (Boolean(ULS) && ULS.enable) {
        ULS.enable = false;
        window.onerror = ULS.OriginalOnError;
        ULS.WebServiceNS = "http://schemas.microsoft.com/sharepoint/diagnostics/";
        try {
            ULS.message = msg;
            if (url.indexOf('?') != -1)
                url = url.substr(0, url.indexOf('?'));
            ULS.file = url.substr(url.lastIndexOf('/') + 1);
            ULS.line = line;
            ULS.teamName = "";
            ULS.originalFile = "";
            ULS.callStack = '<stack>\n' + ULSGetCallstack(oCaller) + '</stack>';
            ULS.clientInfo = '<client>\n' + ULSGetClientInfo() + '</client>';
            ULSSendReport(true);
        }
        catch (e) { }
    }
    if (Boolean(ULS) && Boolean(ULS.OriginalOnError))
        return ULS.OriginalOnError(msg, url, String(line));
    else
        return false;
}

ULSSendExceptionImpl invokes ULSSendReport:

function ULSSendReport(async) {
    ULS.request = new XMLHttpRequest();
    ULS.request.onreadystatechange = ULSHandleWebServiceResponse;
    ULS.request.open("POST", ULSGetWebServiceUrl(), async);
    ULS.request.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
    ULS.request.setRequestHeader("SOAPAction", ULS.WebServiceNS + "SendClientScriptErrorReport");
    ULS.request.send('<?xml version="1.0" encoding="utf-8"?>' 
        + '<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">' 
        + '<soap:Body>' 
        + '<SendClientScriptErrorReport xmlns="' + ULS.WebServiceNS + '">' 
        + '<message>' + ULSEncodeXML(ULS.message) + '</message>' 
        + '<file>' + ULSEncodeXML(ULS.file) + '</file>' 
        + '<line>' + String(ULS.line) + '</line>' 
        + '<stack>' + ULSEncodeXML(ULS.callStack) + '</stack>' 
        + '<client>' + ULSEncodeXML(ULS.clientInfo) + '</client>' 
        + '<team>' + ULSEncodeXML(ULS.teamName) + '</team>' 
        + '<originalFile>' + ULSEncodeXML(ULS.originalFile) + '</originalFile>' 
        + '</SendClientScriptErrorReport>' 
        + '</soap:Body>' + '</soap:Envelope>');
}

Alternatives

An alternative way is to create an own handler that is invoked through an ajax request and that implement an own logic for writing logs to ULS. You can see a neat solution provided by Albert Jan-Shot:

• JavaScript and ULS logging

Not to be confused with SP.ULS.log

There is another utility function for logging: SP.ULS.log, this function logs messages in a new browser window. In my opinion, it is a substitute for console.log, but the good thing is that you have to manually enable it in order to see:

Logging from an app

If you are developing a sharepoint app, you can log app errors easily by using:

SP.Utilities.Utility.logCustomAppError(
       SP.ClientContext.get_current(), errorMessage);