JavaScript中for循环推对象入数组得空数组问题求助（斜率计算）

Hey there! Let's walk through what's causing your empty array and how to fix it, plus touch on your Mocha/TDD question.

The Immediate Problem: Your Loop Condition #

The root cause of your empty slopeValues array is a broken loop condition. Let's break down your loop line:

for (let i = canvas.height / 2; i < canvas.height / 2; i--) {

• You initialize i to 300 (since canvas.height / 2 = 600 / 2 = 300)
• The loop condition checks if i < 300—which is false right from the start. So the loop body never runs at all, hence the empty array.

Fixing the Loop + Calculating Slope 1/2 Points #

First, let's fix the loop to actually execute. Then we'll add logic to generate points with a slope of 1/2.

Slope 1/2 means for every 1 unit you move down (or up) on the y-axis, you move 2 units right (or left) on the x-axis. Let's assume you want points starting from the middle of the canvas (y = 300) and moving down to the bottom, calculating the corresponding x values:

const canvas = { width: 1200, height: 600 };
const slopeValues = [];
const slope = 1/2;
// Start at the middle y-value, loop down to y=0
for (let y = canvas.height / 2; y >= 0; y--) {
  // Calculate x based on slope: Δx = Δy / slope. Since we're moving down, Δy = 300 - y
  const x = (canvas.height / 2 - y) / slope;
  // Only add points that fit within the canvas width
  if (x <= canvas.width) {
    slopeValues.push({ x, y });
  }
}
console.log(slopeValues); // Now this will have populated points!

Alternatively, if you want to iterate over x-values and calculate corresponding y-values (ensuring they stay within the canvas):

const canvas = { width: 1200, height: 600 };
const slopeValues = [];
const slope = 1/2;
const startingY = canvas.height / 2; // Y-intercept at the canvas middle

for (let x = 0; x <= canvas.width; x++) {
  const y = slope * x + startingY;
  // Keep points within the canvas height bounds
  if (y >= 0 && y <= canvas.height) {
    slopeValues.push({ x, y });
  }
}
console.log(slopeValues);

Mocha/TDD Context #

Your empty array issue isn't related to Mocha or TDD itself. The loop logic error would happen in any JavaScript environment. That said, when writing tests:

• Make sure you're initializing variables like canvas consistently in your test cases
• Write assertions that check for the expected number of points, or specific point values (e.g., expect(slopeValues.length).to.be.greaterThan(0) or expect(slopeValues[0]).to.deep.equal({ x: 0, y: 300 }))

内容的提问来源于stack exchange，提问作者Leviathan_the_Great