The TextEncoder.encodeInto() method takes a string to encode and a destination Uint8Array to put resulting UTF-8 encoded text into, and returns a dictionary object indicating the progress of the encoding. This is potentially more performant than the older encode() method — especially when the target buffer is a view into a Wasm heap.

string

A string containing the text to encode.

uint8Array

A Uint8Array object instance to place the resulting UTF-8 encoded text into.

An object, which contains two members:

read

The number of UTF-16 units of code from the source that has been converted over to UTF-8. This may be less than string.length if uint8Array did not have enough space.

written

The number of bytes modified in the destination Uint8Array. The bytes written are guaranteed to form complete UTF-8 byte sequences.

encoder.encodeInto() always puts its output at the start of the array. However, it is sometimes useful to make the output start at a particular index. The solution is TypedArray.prototype.subarray():

const encoder = new TextEncoder();

function encodeIntoAtPosition(string, u8array, position) {
  return encoder.encodeInto(
    string,
    position ? u8array.subarray(position | 0) : u8array,
  );
}

const u8array = new Uint8Array(8);
encodeIntoAtPosition("hello", u8array, 2);
console.log(u8array.join()); // 0,0,104,101,108,108,111,0

To convert a JavaScript string s, the output space needed for full conversion is never less than s.length bytes and never greater than s.length * 3 bytes. If the output allocation (typically within Wasm heap) is expected to be short-lived, it makes sense to allocate s.length * 3 bytes for the output, in which case the first conversion attempt is guaranteed to convert the whole string. Note that the s.length * 3 is rare because the string would have to be packed with some of the few characters that expand into 3 bytes. It is unlikely that long text will exceed s.length * 2 bytes in length. Thus, a more optimistic approach might be to allocate s.length * 2 + 5 bytes, and perform reallocation in the rare circumstance that the optimistic prediction was wrong.

If the output is expected to be long-lived, it makes sense to compute minimum allocation roundUpToBucketSize(s.length), the maximum allocation size s.length * 3, and to have a chosen (as a tradeoff between memory usage and speed) threshold t such that if roundUpToBucketSize(s.length) + t >= s.length * 3, you allocate for s.length * 3. Otherwise, first allocate for roundUpToBucketSize(s.length) and convert. If the read item it the return dictionary is s.length, the conversion is done. If not, reallocate the target buffer to written + (s.length - read) * 3 and then convert the rest by taking a substring of s starting from index read and a subbuffer of the target buffer starting from index written.

Above roundUpToBucketSize() is a function that rounds up to the allocator bucket size. For example, if your Wasm allocator is known to use power-of-two buckets, roundUpToBucketSize() should return the argument if it is a power-of-two or the next power-of-two otherwise. If the behavior of the Wasm allocator is unknown, roundUpToBucketSize() should be an identity function.

If the behavior of your allocator is unknown, you might want to have up to two reallocation steps and make the first reallocation step multiply the remaining unconverted length by two instead of three. However, in that case, it makes sense not to implement the usual multiplying by two of the already written buffer length, because in such a case if a second reallocation happened, it would always over-allocate compared to the original length times three. The above advice assumes that you don't need to allocate space for a zero terminator. That is, on the Wasm side you are working with Rust strings or a non-zero-terminating C++ class. If you are working with C++ std::string, even though the logical length is shown to you, you need to take the extra terminator byte into account when computing rounding up to allocator bucket size. See the next section about C strings.

If the input string contains the character U+0000 in the input, encodeInto() will write a 0x00 byte in the output. encodeInto() does not write a C-style 0x00 sentinel byte after the logical output.

If your Wasm program uses C strings, it's your responsibility to write the 0x00 sentinel and you can't prevent your Wasm program from seeing a logically truncated string if the JavaScript string contained U+0000. Observe:

const encoder = new TextEncoder();

function encodeIntoWithSentinel(string, u8array, position) {
  const stats = encoder.encodeInto(
    string,
    position ? u8array.subarray(position | 0) : u8array,
  );
  if (stats.written < u8array.length) u8array[stats.written] = 0; // append null if room
  return stats;
}

• The TextEncoder interface it belongs to.
• TextEncoder.encode()