Add Float16Array#withAt

src/Float16Array.mjs

@@ -396,6 +396,27 @@ export class Float16Array extends Uint16Array {
     return convertToNumber(this[k]);
   }
 
+  /** @see https://tc39.es/proposal-change-array-by-copy/#sec-%typedarray%.prototype.withAt */
+  withAt(index, value) {
+    assertFloat16BitsArray(this);
+
+    const length = this.length;
+    const relativeIndex = ToIntegerOrInfinity(index);
+    const k = relativeIndex >= 0 ? relativeIndex : length + relativeIndex;
+
+    if (k < 0 || k >= length) {
+      throw new RangeError("Invalid index");
+    }
+
+    const uint16 = new Uint16Array(this.buffer, this.byteOffset, this.length);
+    const proxy = new Float16Array(uint16.slice().buffer);
+    const float16bitsArray = getFloat16BitsArrayFromFloat16Array(proxy);
+
+    float16bitsArray[k] = roundToFloat16Bits(value);
+
+    return proxy;
+  }
+
   /** @see https://tc39.es/ecma262/#sec-%typedarray%.prototype.map */
   map(callback, ...opts) {
     assertFloat16BitsArray(this);

test/Float16Array.js

@@ -667,7 +667,47 @@ describe("Float16Array", () => {
         assert.throws(() => { float16.at(BigInt(0), 0); }, TypeError);
       }
     });
+  });
+
+  describe("#withAt()", () => {
+
+    it("property `name` is 'withAt'", () => {
+      assert( Float16Array.prototype.withAt.name === "withAt" );
+    });
+
+    it("property `length` is 1", () => {
+      assert( Float16Array.prototype.withAt.length === 2 );
+    });
+
+    it("create Array", () => {
+      const float16_1 = new Float16Array([1, 2, 3]);
+      const float16_2 = float16_1.withAt(1, 4);
+
+      assert( float16_1.buffer !== float16_2.buffer );
+      assert.equalFloat16ArrayValues( float16_1, [1, 2, 3] );
+      assert.equalFloat16ArrayValues( float16_2, [1, 4, 3] );
+
+      const float16_3 = float16_1.withAt(-1, 4);
 
+      assert( float16_1.buffer !== float16_3.buffer );
+      assert.equalFloat16ArrayValues( float16_1, [1, 2, 3] );
+      assert.equalFloat16ArrayValues( float16_3, [1, 2, 4] );
+    });
+
+    it("throw Error with invalid index", () => {
+      const float16 = new Float16Array([1, 2, 3]);
+
+      // out of range
+      assert.throws(() => { float16.withAt(5, 0); }, RangeError);
+      assert.throws(() => { float16.withAt(-5, 0); }, RangeError);
+
+      assert.throws(() => { float16.withAt(Symbol(), 0); }, TypeError);
+
+      // Safari 13 doesn't have BigInt
+      if (typeof BigInt !== "undefined") {
+        assert.throws(() => { float16.withAt(BigInt(0), 0); }, TypeError);
+      }
+    });
   });
 
   describe("#map()", () => {