Math.atan2

Math.atan2(y, x) turns a direction into an angle. Give it the vertical and horizontal distances from a starting point to a target, and it returns that direction in radians relative to the positive x-axis. It is the useful choice for a pointer, arrow, dial, or canvas object that must face a changing position. section 9.1.4 section 9.4

Start by subtracting coordinates: deltaX = x2 − x1 and deltaY = y2 − y1. Then call Math.atan2(deltaY, deltaX). The argument order matters: y comes first, x second. Convert the result for CSS rotation with radians * (180 / Math.PI), then use rotate(angleDeg). section 9.4 section 9.6

function getAngle(x1, y1, x2, y2) {
    const deltaY = y2 - y1;
    const deltaX = x2 - x1;
    const angleRadians = Math.atan2(deltaY, deltaX);
    const angleDegrees = angleRadians * (180 / Math.PI);

    return angleDegrees;
}

This helper derives an element-facing angle from a source point and a target point, returning degrees ready for a CSS rotate() transform. section 9.4

Unlike Math.atan(y / x), atan2 keeps both input signs. That preserves the correct quadrant: a target down and left of the source has deltas of −1 and −1, producing −135° rather than the misleading 45° from a ratio alone. Its returned range spans −180° to +180°. section 9.1.4 section 9.4

Use Math.atan2 when an interaction needs direction from a center or source point:

• Rotate an arrow, eye, camera, sprite, or dragged element toward the cursor or another element. section 9.4
• Drive a rotary control by converting pointer coordinates relative to the knob center into an angle, then into a constrained value. section 13.2.2
• Add geometric hover tilt by combining its angle with distance-based intensity and sin/cos rotations. section 10.2.3
• Identify a donut-chart slice by pairing the pointer angle with a distance check against the inner and outer radii. section 14.1.3

Browser coordinates increase downward on the y-axis, so web directions appear clockwise: right is 0°, down is 90°, and down-right is 45°. Math.atan2 provides the vector direction; distance remains a separate calculation, commonly made with the Pythagorean theorem. section 10.1 section 13.5