Function format_value

test/fixtures/wpt/resources/testharness.js:1416–1496

* Convert a value to a nice, human-readable string * * When many JavaScript Object values are coerced to a String, the * resulting value will be ``"[object Object]"``. This obscures * helpful information, making the coerced value unsuitable for * use in assertion messages, t

(val, seen)

     * @returns {string} - A string representation of ``val``, optimised for human readability.
     */
    function format_value(val, seen)
    {
        if (!seen) {
            seen = [];
        }
        if (typeof val === "object" && val !== null) {
            if (seen.indexOf(val) >= 0) {
                return "[...]";
            }
            seen.push(val);
        }
        if (Array.isArray(val)) {
            let output = "[";
            if (val.beginEllipsis !== undefined) {
                output += "…, ";
            }
            output += val.map(function(x) {return format_value(x, seen);}).join(", ");
            if (val.endEllipsis !== undefined) {
                output += ", …";
            }
            return output + "]";
        }

        switch (typeof val) {
        case "string":
            return '"' + val.replace(formatEscapePattern, match => formatEscapeMap[match]) + '"';
        case "boolean":
        case "undefined":
            return String(val);
        case "number":
            // In JavaScript, -0 === 0 and String(-0) == "0", so we have to
            // special-case.
            if (val === -0 && 1/val === -Infinity) {
                return "-0";
            }
            return String(val);
        case "bigint":
            return String(val) + 'n';
        case "object":
            if (val === null) {
                return "null";
            }

            // Special-case Node objects, since those come up a lot in my tests.  I
            // ignore namespaces.
            if (is_node(val)) {
                switch (val.nodeType) {
                case Node.ELEMENT_NODE:
                    var ret = "<" + val.localName;
                    for (var i = 0; i < val.attributes.length; i++) {
                        ret += " " + val.attributes[i].name + '="' + val.attributes[i].value + '"';
                    }
                    ret += ">" + val.innerHTML + "</" + val.localName + ">";
                    return "Element node " + truncate(ret, 60);
                case Node.TEXT_NODE:
                    return 'Text node "' + truncate(val.data, 60) + '"';
                case Node.PROCESSING_INSTRUCTION_NODE:
                    return "ProcessingInstruction node with target " + format_value(truncate(val.target, 60)) + " and data " + format_value(truncate(val.data, 60));