Class to stateless function React - reactjs

Question

I kind of rolled into React when stateless functions where popular so I never experienced the Class approach of it, which is bothering me now..
I'm not sure what this function does:
var keywordMapper = this.createKeywordMapper({
"constant.false": 'false',
"constant.true": 'true',
}, "identifier", true);
Which is called as:
this.$rules = {
"start": [{
token: "constant.numeric", // float
regex: "[+-]?\\d+(?:(?:\\.\\d*)?(?:[eE][+-]?\\d+)?)?\\b"
}, {
token: keywordMapper,
regex: "[a-zA-Z_$][a-zA-Z0-9_$]*\\b"
}, {
token: "keyword.operator",
regex: "\\+|\\-|\\/|\\/\\/|%|<#>|#>|<#|&|\\^|~|<|>|<=|=>|==|!=|<>|="
}]
};
I think it maps the input (what input :s) and returns either 'false', 'true' or 'identifier' as default.
But what if I want to use it within a stateless functional component? Since I can't use this in there.
Any help or explanation on how the 'this' function works is much appreciated.
Greetings,
edit:
The whole useEffect:
useEffect(() => {
const newCompleter = {
getCompletions(editor, session, pos, prefix, callback) {
callback(null, completions);
},
};
const keywordMapper = this.createKeywordMapper({
"constant.false": 'false',
"constant.true": 'true',
}, "identifier", true);
const completionString = completions.map((x) => x.value).join('|');
const session = editor.current.editor.getSession();
session.setMode(`ace/mode/text`, () => {
const rules = session.$mode.$highlightRules.getRules();
if (Object.prototype.hasOwnProperty.call(rules, 'start')) {
rules.start = [
{
token: 'constant.numeric', // float
regex: '[+-]?\\d+(?:(?:\\.\\d*)?(?:[eE][+-]?\\d+)?)?\\b',
},
{
token: keywordMapper(),
regex: '[a-zA-Z_$][a-zA-Z0-9_$]*\\b',
},
{
token: 'keyword.operator',
regex:
'\\+|\\-|\\/|\\/\\/|%|<#>|#>|<#|&|\\^|~|<|>|<=|=>|==|!=|<>|=',
},
];
}
// }
// force recreation of tokenizer
session.$mode.$tokenizer = null;
session.bgTokenizer.setTokenizer(session.$mode.getTokenizer());
// force re-highlight whole document
session.bgTokenizer.start(0);
});
// to extend existing
// addCompleter(myCompleter);
// to override all
setCompleters([newCompleter]);
}, [completions]);
Original class component
ace.define("ace/mode/brms_highlight_rules", ["require", "exports",
"module", "ace/lib/oop", "ace/mode/text_highlight_rules"], function(require, exports, module) {
"use strict";
var oop = require("../lib/oop");
var TextHighlightRules = require("./text_highlight_rules").TextHighlightRules;
var BrmsHighlightRules = function() {
var FalseBool = (
"false"
);
var TrueBool = (
"true"
);
var keywordMapper = this.createKeywordMapper({
"constant.false": 'false',
"constant.true": 'true',
}, "identifier", true);
this.$rules = {
"start": [{
token: "constant.numeric", // float
regex: "[+-]?\\d+(?:(?:\\.\\d*)?(?:[eE][+-]?\\d+)?)?\\b"
}, {
token: keywordMapper,
regex: "[a-zA-Z_$][a-zA-Z0-9_$]*\\b"
}, {
token: "keyword.operator",
regex: "\\+|\\-|\\/|\\/\\/|%|<#>|#>|<#|&|\\^|~|<|>|<=|=>|==|!=|<>|="
}]
};
this.normalizeRules();
};
oop.inherits(BrmsHighlightRules, TextHighlightRules);
exports.BrmsHighlightRules = BrmsHighlightRules;
});
ace.define("ace/mode/brms", ["require", "exports", "module", "ace/lib/oop", "ace/mode/text", "ace/mode/brms_highlight_rules"], function(require, exports, module) {
"use strict";
var oop = require("../lib/oop");
var TextMode = require("./text").Mode;
var BrmsHighlightRules = require("./brms_highlight_rules").BrmsHighlightRules;
var Mode = function() {
this.HighlightRules = BrmsHighlightRules;
this.$behaviour = this.$defaultBehaviour;
};
oop.inherits(Mode, TextMode);
(function() {
this.$id = "ace/mode/brms";
}).call(Mode.prototype);
exports.Mode = Mode;
});
(function() {
ace.require(["ace/mode/brms"], function(m) {
if (typeof module == "object" && typeof exports == "object" && module) {
module.exports = m;
}
});
})();