How to replace characters or words in a string using javascript even with no space between them?

You could create a regular expression with a joined string with pipe and replace found string with the values.

var exchange = function(code) {
        var ref = { one: '1',  two: '2', three: '3' }; 
        return code.replace(new RegExp(Object.keys(ref).join('|'), 'ig'), k => ref[k]);
    };

console.log(exchange("onetwothree"));

You can iterate over number values and replace them.

 var exchange = function(code) {
     var ref = {
       'one' : '1' , 
       'two' : '2' ,
       'three' : '3'
     }; 
   Object.keys(ref).forEach(k => code = code.replace(k, ref[k]))
  return code
 };
 var strg = "onetwothree";
 alert( exchange( strg ) ); // => 123

I think likely the best thing to do is to create a standard parser constructor that goes character by character and then provide it a configuration and a function wrapper.

Constructor

//Parsing input output constructor;
function ParseFor( i, o ) {
    this.str = i;
    this.indexLength = this.str.length - 1;
    this.arr = [ ...i ];
    this.pointer = 0;
    this.output = o;
    this.match = false;
    this.reset = function() {
        this.pointer = 0;
        this.match = false;
    }
    this.next = function( c ) {
        if ( this.arr[ this.pointer ] === c ) {
            if ( this.pointer === this.indexLength ) {
                this.match = true;
            } else {
                this.pointer++;
            }
        } else {
            this.reset();
        }
        return this;
    }
}

Parse Work with RegEx and Constructor

    let parsers = [];
    for ( let [ k, v ] of Object.entries( ref ) ) parsers.push( new ParseFor( k, v ) );
    let parsedStr = str.replace( /./g, ( c ) => {
        let output = "";
        parsers = parsers.map( parser => parser.next( c ) );
        parsers.forEach( parser => {
            if ( parser.match ) {
                parser.reset();
                output = parser.output;
            }
        } );
        return output;
    } );

Parse Function Wrapper

function parseStr( str, ref ) {

//... PARSE CONSTRUCTOR

//... PARSE WORK
    return parsedStr;
}

parseStr( "onetwothree",  {
        'one': '1',
        'two': '2',
        'three': '3'
    });

EXAMPLE:

function parseStr( str, ref ) {

//Parsing input output constructor;
function ParseFor( i, o ) {
	this.str = i;
	this.indexLength = this.str.length - 1;
	this.arr = [ ...i ];
	this.pointer = 0;
	this.output = o;
	this.match = false;
	this.reset = function() {
		this.pointer = 0;
		this.match = false;
	}
	this.next = function( c ) {
		if ( this.arr[ this.pointer ] === c ) {
			if ( this.pointer === this.indexLength ) {
				this.match = true;
			} else {
				this.pointer++;
			}
		} else {
			this.reset();
		}
		return this;
	}
}

//parse work

	let parsers = [];
	for ( let [ k, v ] of Object.entries( ref ) ) parsers.push( new ParseFor( k, v ) );
	let parsedStr = str.replace( /./g, ( c ) => {
		let output = "";
		parsers = parsers.map( parser => parser.next( c ) );
		parsers.forEach( parser => {
			if ( parser.match ) {
				parser.reset();
				output = parser.output;
			}
		} );
		return output;
	} );
	return parsedStr;
}

console.log( parseStr( "onetwothree",  {
		'one': '1',
		'two': '2',
		'three': '3'
	}) );