QJSEngine global string compare

Question

I am using Qt 5.11.2, in my application I am using QJSEngine, in my example I have a script:

    function connect() {
        console.info("-----------");
        if ( strFirstScan.localeCompare("true") == 0 ) {
            console.info("First scan");
            strFirstScan = "false";
            a = eval(a);
            b = eval(b);
            c = eval(c);
        }
        console.info("strFirstScan: " + strFirstScan + ", typeof: " + typeof strFirstScan); 
        console.info("a: " + a + ", typeof: " + typeof a);
        a++;
        console.info("b: " + b + ", typeof: " + typeof b);
        console.info("c: " + c + ", typeof: " + typeof c);
        console.info("-----------");
    }

I have connected this script to a button in the application, when I click the button the connect() function is called by the script. I have registered some globals for use with the script:

    strFirstScan = "true"
    a = 123
    b = "Hello"
    c = {"a":1,"b":"A","c":{"aa":1}}

The output from the script application when the button is clicked is:

    2018-12-28 09:55:10.079663+0000 XMLMPAM[2470:247691] [js] -----------
    2018-12-28 09:55:10.079718+0000 XMLMPAM[2470:247691] [js] strFirstScan: "true", typeof: string
    2018-12-28 09:55:10.079742+0000 XMLMPAM[2470:247691] [js] a: 123, typeof: string
    2018-12-28 09:55:10.079775+0000 XMLMPAM[2470:247691] [js] b: 'hello', typeof: string
    2018-12-28 09:55:10.079804+0000 XMLMPAM[2470:247691] [js] c: {"a":1,"b":"A","c":{"aa":1}}, typeof: string
    2018-12-28 09:55:10.079832+0000 XMLMPAM[2470:247691] [js] -----------

I never see "First Scan" and the types of the variables remains string as it isn't getting to the eval statements.

Why isn't the compare working? I've tried a number of alternatives:

    if ( strFirstScan == "true" ) {

and

    if ( strFirstScan.compare("true") == 0 ) {

None of these are any better, why isn't the compare working?

[Edit] I've modified the script to:

    function connect() {
        console.info("-----------");
        if ( typeof strFirstScan == "string" ) {
            console.info("First scan");
            console.info("strFirstScan: " + strFirstScan + ", typeof: " + typeof strFirstScan); 
            strFirstScan = 0;
            a = eval(a);
            b = eval(b);
            c = eval(c);
        }
        console.info("a: " + a + ", typeof: " + typeof a);
        a++;
        console.info("b: " + b + ", typeof: " + typeof b);
        console.info("c: " + c + ", typeof: " + typeof c);
        console.info("-----------");
    }

With the script like this I get the following in the output:

    2018-12-28 10:22:31.267553+0000 XMLMPAM[2993:335615] [js] -----------
    2018-12-28 10:22:31.267595+0000 XMLMPAM[2993:335615] [js] First scan
    2018-12-28 10:22:31.267629+0000 XMLMPAM[2993:335615] [js] strFirstScan: "true", typeof: string
    2018-12-28 10:22:31.267804+0000 XMLMPAM[2993:335615] [js] a: 123, typeof: number
    2018-12-28 10:22:31.267832+0000 XMLMPAM[2993:335615] [js] b: hello, typeof: string
    2018-12-28 10:22:31.267877+0000 XMLMPAM[2993:335615] [js] c: [object Object], typeof: object
    2018-12-28 10:22:31.267897+0000 XMLMPAM[2993:335615] [js] -----------

However if I add any comparison to the if condition comparing the string against "true" if does not pass into the first scan condition.

[Edit2] I modified my code that creates the global variable 'strFirstScan' to:

    pobjScriptEng->globalObject().setProperty("strFirstScan", QJSValue("true"));

This now resolves the problem and my script:

    if ( strFirstScan == "true" ) {

Works.

Answer 1

I modified my code that creates the global variable 'strFirstScan' to:

pobjScriptEng->globalObject().setProperty("strFirstScan", QJSValue("true"));

This now resolves the problem and my script:

if ( strFirstScan == "true" ) {

This fixes the problem and it now works.

Answer 2

Judging by your program output, strFirstScan is not true, but rather "true".

You can verify that by adding some sort of quotes around it's value in the log output. I.e.: console.info("strFirstScan: '" + strFirstScan + "', typeof: " + typeof strFirstScan);

Here is the snippet to demonstrate your problem:

var yourCase = '"true"';
console.info('value: ' + yourCase + ', type: ' + typeof yourCase + ', len: ' + yourCase.length);
console.info(yourCase === "true");

var youWant = 'true';
console.info('value: ' + youWant + ', type: ' + typeof youWant + ', len: ' + youWant.length);
console.info(youWant === "true");

Why exactly your code behaves as it does, however, is impossible to tell from what you've supplied.