JavaScript function to convert UTF8 string between fullwidth and halfwidth forms

Question

EDIT: Thanks to GOTO 0, I now know exactly what I my question is called.

I need a JavaScript function to convert from UTF-8 fullwidth form to halfwidth form.

Answer 1

Apperently, you want to convert halfwidth and fullwidth form characters to their equivalent basic latin forms. If this is correct, you can do a replacement using a regular expression. Something like this should work:

var x = "！ａｂｃ　ＡＢＣ！";
var y = x.replace(
    /[\uff01-\uff5e]/g,
    function(ch) { return String.fromCharCode(ch.charCodeAt(0) - 0xfee0); }
    );

Where x is your input string and y is the output.

Answer 2

Year 2018 answer

Many years later – and it’s still impossible to find on the Internet a function that does this. So I wrote mine. (Nearly learned Japanese and Korean to get to this point.)

Simple version

Latin range only.

var shiftCharCode = Δ => c => String.fromCharCode(c.charCodeAt(0) + Δ);
var toFullWidth = str => str.replace(/[!-~]/g, shiftCharCode(0xFEE0));
var toHalfWidth = str => str.replace(/[！-～]/g, shiftCharCode(-0xFEE0));

Complete version

Let me know if I missed any character.

(function () {
    let charsets = {
        latin: {halfRE: /[!-~]/g, fullRE: /[！-～]/g, delta: 0xFEE0},
        hangul1: {halfRE: /[ﾡ-ﾾ]/g, fullRE: /[ᆨ-ᇂ]/g, delta: -0xEDF9},
        hangul2: {halfRE: /[ￂ-ￜ]/g, fullRE: /[ᅡ-ᅵ]/g, delta: -0xEE61},
        kana: {delta: 0,
            half: "｡｢｣､･ｦｧｨｩｪｫｬｭｮｯｰｱｲｳｴｵｶｷｸｹｺｻｼｽｾｿﾀﾁﾂﾃﾄﾅﾆﾇﾈﾉﾊﾋﾌﾍﾎﾏﾐﾑﾒﾓﾔﾕﾖﾗﾘﾙﾚﾛﾜﾝﾞﾟ", 
            full: "。「」、・ヲァィゥェォャュョッーアイウエオカキクケコサシ" + 
                "スセソタチツテトナニヌネノハヒフヘホマミムメモヤユヨラリルレロワン゛゜"},
        extras: {delta: 0,
            half: "¢£¬¯¦¥₩\u0020|←↑→↓■°", 
            full: "￠￡￢￣￤￥￦\u3000￨￩￪￫￬￭￮"}
    };
    let toFull = set => c => set.delta ? 
        String.fromCharCode(c.charCodeAt(0) + set.delta) : 
        [...set.full][[...set.half].indexOf(c)];
    let toHalf = set => c => set.delta ? 
        String.fromCharCode(c.charCodeAt(0) - set.delta) : 
        [...set.half][[...set.full].indexOf(c)];
    let re = (set, way) => set[way + "RE"] || new RegExp("[" + set[way] + "]", "g");
    let sets = Object.keys(charsets).map(i => charsets[i]);
    window.toFullWidth = str0 => 
        sets.reduce((str,set) => str.replace(re(set, "half"), toFull(set)), str0);
    window.toHalfWidth = str0 => 
        sets.reduce((str,set) => str.replace(re(set, "full"), toHalf(set)), str0);
})();

/* Example starts here: */
var set = prompt("Enter a couple of comma-separated strings (half or full-width):", 
    ["aouäöü123", "'\"?:", "¢£¥₩↑→", "ｺﾝﾆﾁﾊ", "ﾡﾢￂￃ"].join()).split(",");
var steps = [set, set.map(toFullWidth), set.map(toFullWidth).map(toHalfWidth)];
var tdHTML = str => `<td>${str}</td>`;
var stepsHTML = steps.map(step => step.map(tdHTML).join(""));
var rows = document.getElementsByTagName("tr");
[...rows].forEach((row,i) => row.insertAdjacentHTML("beforeEnd", stepsHTML[i]));

th, td {border: 1px solid lightgrey; padding: 0.2em;}
th {text-align: left;}
table {border-collapse: collapse;}

<table>
    <tr><th scope="row">Input:</th></tr>
    <tr><th scope="row">Full-width:</th></tr>
    <tr><th scope="row">Half-width:</th></tr>
</table>