SOURCE

CanvasRenderingContext2D.prototype.wrapText = function (text, x, y, maxWidth, lineHeight) {
      if (typeof text != 'string' || typeof x != 'number' || typeof y != 'number') {
        return;
      }

      var context = this;
      var canvas = context.canvas;

      if (typeof maxWidth == 'undefined') {
        maxWidth = (canvas && canvas.width) || 300;
      }
      if (typeof lineHeight == 'undefined') {
        lineHeight = (canvas && parseInt(window.getComputedStyle(canvas).lineHeight)) || parseInt(window.getComputedStyle(document.body).lineHeight);
      }

      // 字符分隔为数组
      var arrText = text.split('');
      var line = '';

      for (var n = 0; n < arrText.length; n++) {
        var testLine = line + arrText[n];
        var metrics = context.measureText(testLine);
        var testWidth = metrics.width;
        if (testWidth > maxWidth && n > 0) {
          context.fillText(line, x, y);
          line = arrText[n];
          y += lineHeight;
        } else {
          line = testLine;
        }
      }
      context.fillText(line, x, y);
    };
    function draw() {
      var ctx = document.getElementById('canvas').getContext('2d');
      ctx.font = "16px serif";
      ctx.wrapText("canvas文字换行测试", 0, 16, 100, 16);
    }
    draw();

  <canvas id='canvas' width="100px"></canvas>