fix #2859

packages/jspsych/src/JsPsych.ts

@@ -627,13 +627,14 @@ export class JsPsych {
     };
 
     let trial_complete;
+    let trial_sim_opts;
+    let trial_sim_opts_merged;
     if (!this.simulation_mode) {
       trial_complete = trial.type.trial(this.DOM_target, trial, load_callback);
     }
     if (this.simulation_mode) {
       // check if the trial supports simulation
       if (trial.type.simulate) {
-        let trial_sim_opts;
         if (!trial.simulation_options) {
           trial_sim_opts = this.simulation_options.default;
         }
@@ -658,7 +659,7 @@ export class JsPsych {
         }
         // merge in default options that aren't overriden by the trial's simulation_options
         // including nested parameters in the simulation_options
-        let trial_sim_opts_merged = this.utils.deepMerge(
+        trial_sim_opts_merged = this.utils.deepMerge(
           this.simulation_options.default,
           trial_sim_opts
         );
@@ -685,8 +686,13 @@ export class JsPsych {
     // see if trial_complete is a Promise by looking for .then() function
     const is_promise = trial_complete && typeof trial_complete.then == "function";
 
-    // in simulation mode we let the simulate function call the load_callback always.
-    if (!is_promise && !this.simulation_mode) {
+    // in simulation mode we let the simulate function call the load_callback always,
+    // so we don't need to call it here. however, if we are in simulation mode but not simulating
+    // this particular trial we need to call it.
+    if (
+      !is_promise &&
+      (!this.simulation_mode || (this.simulation_mode && trial_sim_opts_merged?.simulate === false))
+    ) {
       load_callback();
     }