JS 让textarea按tab不切换

让在textarea中按tab符号时输入4个空格, 使用时放于页面末尾即可

HTMLTextAreaElement.prototype.getCaretPosition = function () {
    //return the caret position of the textarea
    return this.selectionStart;
};
HTMLTextAreaElement.prototype.setCaretPosition = function (position) {
    //change the caret position of the textarea
    this.selectionStart = position;
    this.selectionEnd = position;
    this.focus();
};
HTMLTextAreaElement.prototype.hasSelection = function () {
    //if the textarea has selection then return true
    if (this.selectionStart == this.selectionEnd) {
        return false;
    } else {
        return true;
    }
};
HTMLTextAreaElement.prototype.getSelectedText = function () {
    //return the selection text
    return this.value.substring(this.selectionStart, this.selectionEnd);
};
HTMLTextAreaElement.prototype.setSelection = function (start, end) {
    //change the selection area of the textarea
    this.selectionStart = start;
    this.selectionEnd = end;
    this.focus();
};
var textarea = document.getElementsByTagName('textarea')[0];
textarea.onkeydown = function (event) {
    //support tab on textarea
    if (event.keyCode == 9) { //tab was pressed
        var newCaretPosition;
        newCaretPosition = textarea.getCaretPosition() + " ".length;
        textarea.value = textarea.value.substring(0, textarea.getCaretPosition()) + " " + textarea.value.substring(textarea.getCaretPosition(), textarea.value.length);
        textarea.setCaretPosition(newCaretPosition);
        return false;
    }
    if (event.keyCode == 8) {
        //backspace
        if (textarea.value.substring(textarea.getCaretPosition() - 4, textarea.getCaretPosition()) == " ") {
            //it's a tab space
            var newCaretPosition;
            newCaretPosition = textarea.getCaretPosition() - 3;
            textarea.value = textarea.value.substring(0, textarea.getCaretPosition() - 3) + textarea.value.substring(textarea.getCaretPosition(), textarea.value.length);
            textarea.setCaretPosition(newCaretPosition);
        }
    }
    if (event.keyCode == 37) { //left arrow
        var newCaretPosition;
        if (textarea.value.substring(textarea.getCaretPosition() - 4, textarea.getCaretPosition()) == " ") {
            //it's a tab space
            newCaretPosition = textarea.getCaretPosition() - 3;
            textarea.setCaretPosition(newCaretPosition);
        }
    }
    if (event.keyCode == 39) {
        //right arrow
        var newCaretPosition;
        if (textarea.value.substring(textarea.getCaretPosition() + 4, textarea.getCaretPosition()) == " ") {
            //it's a tab space
            newCaretPosition = textarea.getCaretPosition() + 3;
            textarea.setCaretPosition(newCaretPosition);
        }
    }
}