Concatenate values

To simplify the previous day’s entry, we have text_without_smiley with the result of the $concat operation, which combines the first and second substrings from the previous day’s journal.

// test data
db.getCollection("unicode_demo").insertMany([
	{ label: "with_emoji", text: "Hi☺!" },
	{ label: "multi_byte_chars", text: "Héllo☺!" },
]);

db.getCollection("unicode_demo").aggregate([
	{
		$project: {
			label: 1,
			text: 1,
			text_without_smiley: {
				$let: {
					vars: {
						smileyIndex: { $indexOfCP: ["$text", "☺"] },
						textLen: { $strLenCP: "$text" },
					},
					in: {
						$concat: [
							{ $substrCP: ["$text", 0, "$$smileyIndex"] },
							{
								$substrCP: [
									"$text",
									{ $add: ["$$smileyIndex", 1] },
									{ $subtract: ["$$textLen", { $add: ["$$smileyIndex", 1] }] },
								],
							},
						],
					},
				},
			},
		},
	},
]);

Output:

[
  {
    _id: ObjectId('69b6987355ed455452544ca7'),
    label: 'with_emoji',
    text: 'Hi☺!',
    text_without_smiley: 'Hi!'
  },
  {
    _id: ObjectId('69b6987355ed455452544ca8'),
    label: 'multi_byte_chars',
    text: 'Héllo☺!',
    text_without_smiley: 'Héllo!'
  }
]