Source

File: wp-includes/rest-api/class-wp-rest-request.php

protected function parse_json_params() {
		if ( $this->parsed_json ) {
			return true;
		}

		$this->parsed_json = true;

		// Check that we actually got JSON.
		$content_type = $this->get_content_type();

		if ( empty( $content_type ) || 'application/json' !== $content_type['value'] ) {
			return true;
		}

		$body = $this->get_body();
		if ( empty( $body ) ) {
			return true;
		}

		$params = json_decode( $body, true );

		/*
		 * Check for a parsing error.
		 */
		if ( null === $params && JSON_ERROR_NONE !== json_last_error() ) {
			// Ensure subsequent calls receive error instance.
			$this->parsed_json = false;

			$error_data = array(
				'status'             => WP_Http::BAD_REQUEST,
				'json_error_code'    => json_last_error(),
				'json_error_message' => json_last_error_msg(),
			);

			return new WP_Error( 'rest_invalid_json', __( 'Invalid JSON body passed.' ), $error_data );
		}

		$this->params['JSON'] = $params;

		return true;
	}