index.html

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body style="height: 100%">
    <textarea name="" id="input" cols="120" rows="20">++++++[>++++++++++<-]>+++++.</textarea>
    <br>
    <br>
    <button onClick="compile()">Compile</button>
    <br>
    <br>
    <p id="output" style="width: 70%; border: solid; height:70%; overflow: auto; padding: 5px">
    </p>
    <script>
        let memory
        let pointer
        let out
        function compile(){
            //alert((new DOMParser).parseFromString('<!doctype html><body>' + document.getElementById('input').innerHTML, 'text/html').body.textContent)
            memory = []
            pointer = 0
            out = document.getElementById('output')
            out.innerHTML = ''
            eval(document.getElementById('input').value)
        }

        function eval(input) {
            for (let char in input) {
                switch (input[char]) {
                    case '+':
                        if (memory[pointer] == null) memory[pointer] = 0
                        memory[pointer] += 1
                        break
                    case '-':
                        if (memory[pointer] == null) memory[pointer] = 0
                        if (memory[pointer] > 0) memory[pointer] -= 1
                        break
                    case '.':
                        out.innerHTML = out.innerHTML + String.fromCharCode(memory[pointer])
                        break
                    case '>':
                        pointer += 1
                        break
                    case '<':
                        if (pointer > 0) pointer -= 1
                        break
                    case '[':
                        loopContent = ""
                        for (c of input.substring(parseInt(char) + 1, input.length)) {
                            if (c == ']') break
                            loopContent += c
                        }
                        while (memory[pointer] > 1) {
                            eval(loopContent)
                        }
                        break
                    case ',':
                        memory[pointer] = parseInt(prompt('test'))
                }
            }
        }
    </script>
</body>

</html>