4 examples of 'how to check if user already exists in database in jsp' in JavaScript

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function checkUserNotExists() {
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    // Send request only if 'Input' is not empty.
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    if ($("#username-input").val()) {
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        $.ajax({
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            url: "is-user-exists",
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            type: "GET",
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            data: {username: $("#username-input").val()},
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            success: function result(json) {
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                if (json) {
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                    document.getElementById("user-exists").style.display = "block";
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                }
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                else {
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                    document.getElementById("user-exists").style.display = "none";
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                }
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            }
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        });
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    }
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}

6
function userExists(email) {
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	var results = api.db.query('users', {
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		email: email
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	});
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	if (results.length === 1) {
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		return true;
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	}
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	return false;
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}

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export function checkUsernameAlreadyExists(data) {
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    let errors = {};
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    return User.query({
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        where: { username: data.username }
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    })
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        .fetch()
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        .then((user) => {
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            // User found in database
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            if(user) {
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                if (user.get('username') === data.username) {
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                    errors.username = 'This username already exists, please choose different one.';
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                }
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            }
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            return {
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                errors,
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                isValid: isEmpty(errors)
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            }
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        });
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}

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export async function checkUsernameExists(username) {
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    const conn = await pgClient.connect();
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    try {
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        const result = await conn.query(
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            `SELECT username FROM ${PG_TN_USERS} WHERE username = $1`,
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            [username.toLowerCase()],
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        );
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        if (result.rows.length > 0) {
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            return true;
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        } else {
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            return false;
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        }
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    } catch (ex) {
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        throw ex;
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    } finally {
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        conn.release();
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    }
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}